“When I shot the rafidi it was easy. He was kneeling in front of me, sobbing. He was a big man: his body was shaking. I thought ‘why are you crying, you are a killer yourself, you are going to have a quick death’. I just pulled the trigger, and it was over. I didn’t feel any pity. I didn’t feel any pride. I was just doing my job.”

The shooting in the town of Manbij was the first execution of a prisoner that Abu Mutassim, an experienced fighter for Isis, had carried out. He tried to justify the murder by saying that the man whose life he took was not just an enemy but a member of the Shabiha, a brutal Bashar al-Assad regime militia. The final punishment was justified against these types of rafidis – a pejorative Sunni term for Shias and Syria’s ruling Alawites – who committed atrocities, he argued.

I met Abu Mutassim in northern Syria, across the border from Turkey. He was one of three men who had deserted from Isis and faced an uncertain future. The jihadis are in retreat, their “caliphate shrinking”, but that did not mean they could not still exact vengeance on traitors: fighters have been executed just on the suspicion of planning to leave.

A vengeful Isis was not the only threat to these fleeing men. The families and comrades of those they have killed in the course of the brutal civil war, from other rebel groups and tribal militia, as well as forces of the regime would also exact retribution if they had a chance to do so.

It was perhaps this knowledge which made Abu Mutassim appear fatalistic and be more candid than most other fighters who had fled the ranks of Islamist groups in Syria and Iraq. The two young men with him, Muslims from Belgium, had volunteered interesting information including that the hierarchy of Isis was paranoid about infiltration by Western intelligence services. At the same time, both had been anxious to repeatedly stress that they had played no part in the acts of savagery for which Isis has become a byword.

I spoke to Abu Mutassim, away from his companions, in a village near Jarablus, part of an area under the control of the Turkish military sent by President Recep Tayyip Erdoagan to carve out a “security zone” in Syria. The force, with aircraft, armour and artillery, is backing rebels on a dash for Raqqa, the Isis “capital”, while also battling American backed Kurdish fighters.

“If you listen to all these people who had left Daesh [Isis], you’d think all they were doing was cooking or driving. I had no idea we had so many cooks and drivers! I wonder why we didn’t open lots of restaurants and a taxi service,” Abu Mutassim laughed.

“I don’t blame anyone for saying things to stay alive, especially the ones who came from other countries. They want to go back there. May be some of them will do what the emirs [leaders of Isis] have told them to do and carry out attacks when they return: maybe they will just go back to their lives. But they don’t want to get arrested.”

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Like other former fighters, 30-year-old Abu Mutassim had trimmed the long beard he wore in the photographs he showed me from his jihadi time, and had disposed of his combat uniform. He and his companions claimed that they had thrown away their weapons. My translator, overhearing snatches of conversation, thought the guns had been hidden away instead. The rugged AK-47 Kalashnikovs can be buried with minimal wrapping without too much risk of damage or rusting, unlike sophisticated and expensive Western models.

With his hair newly cut and gelled, wearing a T-shirt with a baseball motif and jeans, Abu Mutassim looked almost relaxed, although he kept glancing up every time someone came into the chai shop where we had met. It was a relief, he said, to be able to smoke openly: the stern Salafist creed of Isis decreed that those breaking the ban on tobacco would face jail or public flogging.

In this appearance, on a sunny autumn day, Abu Mutassim, with his square, open face and alert eyes, did not seem that different from the man I had met in Aleppo four years ago; someone in our realm of normality, someone one could talk to. This was during a battle lasting the summer when the rebels seemed close to capturing Syria’s largest city and commercial centre, and then marching on Damascus.

In Pictures: The crisis unfolding in Syria Show all 30 1 /30 In Pictures: The crisis unfolding in Syria In Pictures: The crisis unfolding in Syria A Syrian family arrives at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods Getty In Pictures: The crisis unfolding in Syria A Syrian woman, fleeing violence in the restive Bustan al-Qasr neighbourhood, reacts as she stands with her children in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past resident fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood , after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past residents fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria A Syrian pro-regime fighter speaks with a child, as residents flee violence in the restive Bustan al-Qasr neighbourhood. Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops AFP/Getty Images In Pictures: The crisis unfolding in Syria Smoke rises as seen from a governement-held area of Aleppo, Syria Reuters In Pictures: The crisis unfolding in Syria Syrian soldiers targeting rebels-held areas in the eastern neighborhoods in Aleppo, Syria. According to media reports, the army is now holding on 99 percent of Aleppoís eastern neighborhoods EPA In Pictures: The crisis unfolding in Syria Syrian pro-government forces patrol Aleppo's eastern al-Salihin neighbourhood after troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian soldiers rest following the battle at al-Sheik Saeed neighborhood in Aleppo, Syria EPA In Pictures: The crisis unfolding in Syria A Syrian pro-government fighter walking past closed shops in the Bab al-Nasr district of Aleppo's Old City. Once renowned for its bustling souks, grand citadel and historic gates, Aleppo's Old City has been rendered virtually unrecognisable by some of the worst violence of Syria's war Getty In Pictures: The crisis unfolding in Syria The crucial battle for Aleppo entered its 'final phase' after Syrian rebels retreated into a small pocket of their former bastion in the face of new army advances. The retreat leaves opposition fighters confined to just a handful of neighbourhoods in southeast Aleppo, the largest of them Sukkari and Mashhad Getty In Pictures: The crisis unfolding in Syria Syrian civilans arrive at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods. Syria's government has retaken at least 85 percent of east Aleppo, which fell to rebels in 2012, since beginning its operation Getty In Pictures: The crisis unfolding in Syria Syrian civilians flee the Sukkari neighbourhood towards safer rebel-held areas in southeastern Aleppo Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances. The fall of Aleppo would be the worst rebel defeat since Syria's conflict began in 2011, and leave the government in control of the country's five major cities Getty In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee Aliya inside the tent where she lives with her husband and ten children in a camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee women and children outside the entrance to their tents in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee woman outside the entrance to the tent where her family live, in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A vehicle drives past a mosque at night in Idlib, Syria. Picture taken with a long exposure Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria The night sky is seen through damaged windows in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled area of Maaret al-Numan in Idlib province, Syria Reuters

But the opposition fell out among themselves: a recurring theme in Syria’s uprising. The West, having encouraged the people to rise up, gave no effective support on the ground to the moderate rebels. Jabhat al-Nusra and then Isis came to the forefront, giving President Assad the enemy he had always wanted, allowing him to claim that he was facing the scourge of terrorism like so many others in the international community.

Abu Mutassim had been a grain trader in Aleppo, living with his family in a village to the west of the city. He was among the thousands who had taken part in protest marches. When the demonstrations were suppressed, with great brutality, by the regime, he shut up his shop and took up the gun. The rebel band Abu Mutassim was with broke up as the fighting came to a stalemate. He went to live for a while across the Turkish frontier, drifted back to Syria, joined another group, Liwa al-Tawhid, and then Isis.

We talked about Abu Mutassim’s family. His mother had died of cancer two years ago, the supply of drugs that was keeping her alive having stopped in the chaos of the conflict. One brother was arrested by the regime’s secret police and had not been seen since. His wife, seven-year-old son, a sister, two other brothers and 76-year-old father were staying at the family farm. He did not want their whereabouts to be known as they would be targets because of him. “I have kept in touch, but I can’t meet them. Of course I miss my family, my son most of all, but it’s for their own safety,” he said.

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How did the views he talked about, with some passion – at the start of the revolution – of a tolerant and democratic post-Assad Syria, change so much? I asked. He struggled to answer. “It was a very confusing time, we were very angry with what was happening, I wanted to be with those who were the best fighters against Bashar, had the best weapons, So that’s why...” he shrugged.

“I am a Muslim, I pray. But Daesh did not think I was religious enough, so they sent me to a rehabilitation programme, but that was not very long because they needed men for operations. I made my bay’a [pledge of allegiance to Abu Bakr al-Baghdadi, the “caliph” of Isis] and then I was a member. All I wanted to do was to hit back at the regime.”

“One reason some of us joined was we were angry with Americans, with the British, the French. They wanted us to fight against Bashar [al-Assad], to bring him down, they promised us help. How many people died waiting for that help to arrive? What did America and Europe do when Assad dropped barrel bombs on hospitals, schools? So us joining Daesh was a form of revenge.”

Joining Islamist extremists after being let down by the West is now a familiar theme among many fighters. There may well be truth in this – but how did that explain the beheadings, burnings, shootings and rapes of Syrians and Iraqis committed by Isis?

“A lot of people did not agree with those things: that is the main reason why we left. They twisted Sharia and persecuted people when they should have been devoting their time against Assad,” said Abu Mutassim. “I saw them cut off the heads of two women in Deir Ezzor saying they were witches, it was shocking. I knew then they were going down the wrong path, the people were hating them.

“But do you know who are the most cruel in this? It is the foreign fighters. They were always pretending they were more religious than us, they were informing on other fighters, on the people, Syrians. There is a lot of suffering because of these foreigners.”

Abu Mutassim was not surprised that Mohammed Emwazi, or “Jihadi John”, from London and a team of fellow British nationals were made jailers and executioners of journalists and aid workers held as hostages. “You never saw these people much in the frontline, they always tried to get into the amniyat [Isis’s ‘security service’] so they could torture and punish people,” he said.

I wanted to talk about Abu Mutassim’s knowledge about the inner workings of Isis. But he wanted to dwell on death. “All sides have killed people in this war. Hundreds of civilians have been killed by the airplanes of the Americans, the Russians and the British. Bashar’s people have carried out massacres. And what about horrible things being done by fighters supported by Europe and America? We know someone who ate a human body, don’t we?”

The cannibal was Abu Sakkar, regarded as a “moderate” rebel commander by the West and a highly effective one at that. Omar al-Farouq – his khatiba or battalion – was praised for combating extremists. They had even arrested and executed the leader of a group of foreign Islamists. We, a group of journalists, had met him at the time, and got on with him.

Khalid a-Hammad, which was his real name, had killed Mohammed al-Absi, himself a murderer who was believed to be responsible for the kidnapping of the journalists John Cantlie and Jeroen Oerlemans – British and Dutch – in 2012. They were subsequently rescued. Mr Cantlie went back to Syria later that year with another freelance, Jim Foley. Both were captured and handed over to Isis: Jim was beheaded, John remains a captive forced to make jihadist propaganda videos.

It was also a video that made Abu Sakkar’s acts particularly notorious, even by the standards of Syria’s war. He was filmed eating the freshly cut lungs of a dead regime soldier, shouting, while mutilating a corpse: “I swear to God we will eat your hearts and your livers, you soldiers of Bashar the dog…” A gunman alongside grinned: “It looks like you’re carving him a Valentine’s heart.”

“So, even in all this violence, there are differences…” muttered Abu Mutassim. Was he saying there were degrees of evil one must almost grade what happens in a conflict like Syria? I asked. “Maybe, maybe,” nodded Abu Mutassim when the translator explained. “Abu Saker is dead now I think. Maybe that is what will happen to us all.”

Perhaps Abu Mutassim really was not long for this world. But then here he was, sipping tea, in an area where he should be, at least, arrested. But he was, the whisper went, under the protection of a rebel group, one that had been backed by Turkey and Saudi Arabia, fighting Isis.

Their target is al-Bab, which straddles the route to Raqqa. Sixteen Turkish soldiers were killed last week in the battle for the town, the biggest loss by President Erdogan’s forces since the intervention. Two captured special forces troopers were burned to death, their murders filmed and broadcast by the jihadis.

Abu Mutassim knew a lot about al-Bab – he was based there for while – and he seemed to know a lot about the structure and personnel of broader Isis. Such as how the chief of the amniyat in al-Bab was executed after being accused of being a British spy; which roads in and out have, and have not, been mined; where the bodies are (literally) buried; the favourite BMW the emir of al-Bab could be found driving; who may have organised the death of Abu Rahman al Tunisi, the head of Isis’s “foreign intelligence service”: the part emir of Deir Ezzor, Abu Dhar the Iraqi, played in the torture and murder FSA [Free Syrian Army] fighters.