Unlike his fellow prisoners, Hamoud Saleh Hamed does not wear leg shackles when he is pushed on a spanking new wheelchair into the Syrian prison governor’s guest room at the Mezze military jail. Impossible, since his trousered right stump and the missing lower half of his left leg mean that the 37-year old Saudi maths teacher from Mecca cannot escape by throwing himself from the second floor window. But he speaks with great calmness, a very intelligent man with long hair and a whispy beard who recounts with frightening lucidity his life as a Nusrah Front mortar platoon commander and an executioner who fired four shots into the head of a Syrian government "collaborator".

Hamed talks about fate, and says he regrets killing the 50-year old man whose name he never knew, but he recounts with some pride his weapons’ training and his role in the bloody July 2012 Battle of Damascus when thousands of Nusrah and allied rebels vainly tried to capture the city from its Syrian army defenders. He even used Google to plot his artillery ranges across the city but lived to witness the Syrian civilians he came to ‘save’, begging him to leave the country along with his fellow fighters. “When the people wanted us to go, it gave me great pain,” Hamed said, rubbing what was left of his left leg.

“Frankly,” the governor’s balding intelligence officer admitted before Hamed was brought into the room, “I’m very surprised they are going to let you meet this man.” But I could well understand why "they" did. Hamed was a Saudi who fought against the Syrian regime alongside other Wahabi Sunni Saudis, saying he was misled about the Syrian war by the internet and the Qatari ‘al-Jazeera’ channel, describing in detail how he was groomed in Saudi Arabia for his ‘jihad’ by a friend who arranged his passage through Turkey into the Syrian killing fields. Terribly wounded, he was captured by Syrian troops last year while trying to flee to Idlib.

Hamoud Saleh Hamed, a Saudi maths teacher mutililaed by war, awaits the death sentence in a Syrian jail. Photograph: Nelofer Pazira

Hamed smiled a lot during his interview in the early hours of the morning, his words occasionally interrupted by the rumble of shellfire from the darkness outside. Like the other prisoners, he had broken his Ramadan fast the previous evening. The governor and his security officer left the room at our request and Hamed said he wanted to talk, even when we told him he did not have to speak with us and could just relax and drink the glass of orange juice beside him. He had never met a foreign journalist before but he enjoyed talking, he said. Ten months in a Syrian jail did not provide much opportunity to chat to anyone. He had not been harmed in prison, he insisted, and “the stories I had heard about what happened here were untrue.” Readers must make of this statement what they will.

Many Islamists have expressed their fascination with mathematics, but Hamed said he gained his MA because he had good teachers in “respected Mecca”, where he married a Saudi woman and was father of a son and four daughters. His father, now dead, had been muezzin at a Mecca mosque, calling Muslims to prayer five times a day. “The Syrian war had started and there were many protests,” he said. “The internet and the television channels like al-Jazeera said that the Syrian people were asking for help. Jihadi slogans spread and the jihadis outside Syria began to call for help to defend the people and stop the brutality of the Assad regime. The idea of going to Syria and to participate grew in my mind. Some sheikhs and religious leaders in Saudi Arabia spoke on television and in the mosques and encouraged us. Jihad is like a duty in Islam.”

A Syrian friend in Mecca, whom he called ‘Abdul-Rahman al-Syri’ (Abdul- Rahman from Syria), organised his Saudi Airlines flight to the Turkish city of Antakya although his departure was kept secret for fear that the Saudi intelligence service would find out. “They were refusing to allow people to go to Syria,” Hamed said, “but for political reasons, nothing to do with Islamic ‘sharia’. There was coordination so that when I arrived a man called Abdul-Rahman abu Hajar took me and several others to an apartment for two nights and then took us to the border where, after a Turkish patrol had passed, we climbed through a hole in the fence and I met another Saudi called Abu Rawaha. He transported us to the Syrian town of Atme [in Idlib province] in a car.”

There was a ‘guest house’ in Atme, Hamed said, where their identity papers and passports were taken from them for "safekeeping". After two more days, they were taken to a Nusrah-al-Qaeda military training camp where they were taught to use Kalashnikov rifles, RGP B-7 anti-tank rocket launchers and mortars. The teachers were Egyptian, their instructor Turkish. After a month, Hamed was sent to the countryside of Deir-ez-Zour with seven men, an Egyptian, a Qatari and five Saudis, where they met the Nusrah leader of eastern Syria, ‘Emir’ Abu Maria al-Qahtani, who would later be demoted by the Nusrah leadership during a dispute over relations with Isis.

“After we met,” Hamed said, “we pledged our loyalty and obedience and prayed that we would accept good times and bad times and would not question the orders of our commander as we sought to see those who are infidels in the sight of God. We were taken to the city of Deir ez-Zour which was under siege and there was fierce fighting. Then, in a convoy of cars, vans and jeeps, on roads and through the desert, Hamed says he was taken with seven more men to the Damascus suburb of Ghouta where, staying in “a great house” he was told to train on mortars for a month, after which, “to prepare for a very big battle on Damascus”, he was sent to the area of Jobar.

“One of our leaders, ‘Abu-Bakr al-Jordani’ (Abu Baker from Jordan), pin-pointed our targets. I was to fire at the Abbasin stadium and the Panorama war memorial area. I was given the target points from Google maps. The idea was to separate the defending Syrian soldiers, to confuse them. The plan was for seven suicide car bombers to enter Damascus. Zero hour for me was 9.0am on 15 July [2012] and we started shelling.” Hamed admits he did not know who was in his target areas, but says that his own home-made mortars began to explode and one was hit by Syrian army fire and he was eventually forced to escape as government forces advanced. “There had been mistakes by our leaders, one of the car bombs exploded on the road into the city.”

As his men shelled al-Ghouta – which was now under siege by the army – Hamed retreated yet again, to al-Ateibi and then to the village of Marj al-Sultan where a Nusrah man invited him to marry his niece as a second wife. She would later bear him two children. Near the village, Hamed was driving a vehicle in the company of his brother-in-law when a Syrian army rocket hit the vehicle, blowing off his left leg and severing his right leg at the knee. There followed months of medical operations in makeshift rebel hospitals and four days of surgery.

Hamed smiled at me as he said this. Was it worth it, I asked? He sighed and was silent for almost half a minute. “It was fate,” he said. “I will tell you about an incident. There was a man in the Ghouta area who had been caught signaling targets to the Syrian army. A religious judge, a mufti, condemned him to death. They asked me if I would kill him. It was outside, and the man was kneeling on the ground. He was about 50. He confessed before several of the leaders who were there. Then I shot him in the back of the head. I did not know his name. The others told me to keep shooting and I shot him three more times. I felt nothing. If this man was a Muslim and had made a sin, when I killed him I purified him from his sins.”

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Hamed talked again about fate, of further months of medical operations and then of his regrets. “It was fate,” he said again. “In Ghouta, the siege was worse and people were very hungry and most civilians hoped the regime would come back to their area. When the people wanted us to go, it gave me great pain. The people who we wanted to help didn’t want us any more. And it was painful to me when fighting broke out between the different rebel groups, between Muslims, between the ‘Free Syrian Army’ and Nusrah and Daesh [Isis].”

Hamed hopes that one day he might be released from prison, to go to Turkey or another state to live with his second wife and children. “I telephoned my first wife in Saudi Arabia after I was wounded,” Hamed said. “She said she was glad that in my plight I had someone to look after me.” But he was captured trying to escape. Now, in prison, he was well treated, he said. “I was told lies about what happened here.”