Tripoli, Lebanon - Abu Huraira is haunted by memories of shells falling from the sky, ripping to pieces people trying to bury their loved ones in a Syrian graveyard. He is unnerved by the mental images of mutilated bodies, and of bones laid bare in his rebel comrades' deep wounds.

But the 24-year-old Lebanese man is nevertheless planning to return to Syria to fight in the civil war against its government, led by President Bashar al-Assad.

The muscular man - who follows Salafism, a strict form of Sunni Islam - recalls the bloody battles he fought in the Syrian city of Qusair last summer. Abu Huraira had crossed the border with 16 of his friends to fight against Assad's forces - but more importantly against Hezbollah, the Lebanese Shia armed group that sent fighters to boost Assad's troops in Qusair.

"When Hezbollah openly declared they were supporting the criminal Assad regime in butchering Sunnis, I thought it was my duty to help my brothers in Syria," Abu Huraira told Al Jazeera, sitting at an outdoor coffee shop in his hometown, Tripoli.

We could not help the injured screaming in pain. Some were begging us to shoot them dead to stop their misery, but we asked them to be patient. People were dying as they dug the graves of their friends. The situation was unimaginable. - Abu Huraira, Lebanese Sunni fighter

"I travelled to Qusair, then called my mother and told her: 'Either we come back victorious or as martyrs,'" said the man with a bushy red beard.

Scores of Sunnis from Lebanon, and especially Sunni-majority Tripoli, sneaked into Syria to fight alongside overwhelmingly Sunni rebels in Qusair. Hezbollah, meanwhile, sent thousands of Shia fighters to support Assad's military. The Syrian president and many in his government come from the Alawite sect, an offshoot of Shia Islam.

'Shia Crescent' fears

"It was an ideological battle, a holy war. Hezbollah invaded Syria as part of the Iranian-backed plan to form the Shia Crescent," Abu Huraira said, referring to a swathe of the Middle East stretching from Iran to Lebanon, where Shia Muslims are most numerous.

Many Sunnis worry that Iran, the most powerful Shia-majority country, wants to expand its influence across the so-called Crescent, in Iraq, Syria, and Lebanon.

Hezbollah, meanwhile, accuses people such as Abu Huraira of being "terrorists" and takfiris, or Muslims who are intolerant of others' religious views. The Shia group says it is fighting the rebels in Syria because they pose a threat to both Muslims and Christians in Lebanon.

Following months of deadly battles, Syrian troops backed by Hezbollah eventually recaptured Qusair last June, cutting off major rebel supply routes from Lebanon.

The surviving fighters fled to nearby villages. Of Abu Huraira's friends, only two managed to return to their homes in Lebanon. The rest either died or moved on to fight in other parts of Syria. Abu Huraira left Qusair with wounds to his arm and knee, but said it was psychological exhaustion that forced him and his fellow fighters to withdraw.

"Hezbollah went into Syria under a heavy cover of shelling by the Syrian regime," he said. "We could not help the injured screaming in pain. Some were begging us to shoot them dead to stop their misery, but we asked them to be patient. People were dying as they dug the graves of their friends. The situation was unimaginable."

'Religious duty'

About 30 fighters from Tripoli died fighting in Qusair, according to Sheikh Salem al-Rafei, a prominent cleric in the city and staunch supporter of Syria's rebels. It is a relatively high number for a city such as Tripoli, with a population of more than 500,000.

Initially, Lebanese who sympathised with the rebels went to Syria on their own accord. But during the Qusair battles, clerics led by Rafei called on people to cross the border to fight Assad's forces and Hezbollah, and coordinated the entry of three battalions into the war-torn country.

Abu Huraira , 24, fought in the Syrian city of Qusair last summer [Al Jazeera]

"Before Qusair fell, we saw videos of civilians there asking for rescue. It was a religious duty to fight in Syria," Rafei told Al Jazeera from his home in Tripoli.

But unlike the well-trained Iranian-backed Hezbollah fighters, the Sunni fighters were disorganised, lacking strong leadership and sustained funding.

Abu Huraira, who had worked as a hospital security guard before going to Syria, had to buy his own gun. After he was wounded, he struggled to cover the cost of surgery.

"Hezbollah, on the other hand, takes care of its members," he told Al Jazeera. "When one of their fighters travels to Syria, they take care of his family and if he is injured, they provide him with the best healthcare."

Abu Huraira, who initially had little fighting experience, began manning a rebel checkpoint in Qusair and later developed skills to attack military positions and join the fighting on the frontlines. He has several photos and videos on his phone, showing him scrambling over a captured tank and posing with a gun.

Seeking revenge

Other than his declared ideological motivation for going to Syria, Abu Huraira was also seeking revenge. His hatred against the Syrian government goes back to the Lebanese civil war in the 1980s, when Syrian troops - sent by Bashar's father Hafez al-Assad and aided by local Alawites in Tripoli - crushed Sunnis seen as being close to Palestinian groups fighting in Lebanon.

Abu Huraira's father, who was a member of an armed group called the al-Tawheed al-Islami Movement, was killed at a Syrian checkpoint in Tripoli in the early 1990s.

"As Sunnis, we lived through the injustice and criminality of the Syrian regime ourselves, so we know what Syrians are facing now. This is why we cannot live unmoved by what is happening there," he said.

Abu Huraira has vowed to return to Syria to fight [Al Jazeera]

Many Lebanese Sunnis in Tripoli say they feel alienated in their own country, and have developed stronger ties to Sunnis in Syria's border towns, rather than with more secular Sunnis living in Lebanon's capital, Beirut.

After Lebanon's 15-year-long civil war ended in 1990, successive governments focused on rebuilding and investment in Beirut, leaving other cities struggling with poverty and unemployment.

"When I go to Beirut I find people scared of my beard, even people in Sunni neighbourhoods. I struggle to find a bearded man or a niqabi [veiled] woman. But in Qusair, I felt I was in my own environment. I feel that they are Muslims," Abu Huraira said.

He is preparing to go back to Syria next week. Despite his aching knee, he plans to cross Lebanon's Bekaa Valley into Qalamoun, a border area where clashes are raging between rebels and Syrian troops backed by Hezbollah.

"I will go to any place in Syria where Hezbollah is fighting so I can fight them back," he told Al Jazeera. "I am a terrorist and an infidel to Hezbollah. There is no flattery and concessions. I will fight them till the very end."