EXCLUSIVE: He had a ringside seat for the entire, bloody rise of ISIS, and by his own count killed dozens of innocent men, women and children. Now facing likely execution, Thahir Sahab Jamel disavows the black-clad Islamist army, but his Kurdish jailers say they’ve heard it all before.

In a jailhouse interview with FoxNews.com in the Kurdish-held city of Kirkuk, Jamel, 27, detailed how he joined Islamic State in 2013, served as a foot soldier in the takeover of Mosul a year later and, he claims, eventually became disillusioned with the dark vision of his fellow fighters.

“At the beginning, ISIS told us we would all go to heaven,” Jamel said, speaking under the watchful eyes of Kirkuk police guards. “But now that I am in prison it means I am going to the fire. I am going to hell.”

Handcuffed and partially masked, Jamel, who has been in solitary confinement since his arrest two and a half months ago, said he joined the terror group like many other young Sunni Muslim men opposed to the Shia-led government in Baghdad.

“A man named Salam talked to me and got me connected to ISIS. He told me I needed to be a jihadist and fight the Shia government. He convinced me to fight the government,” Jamel said. “I started getting involved as they were planning operations to begin in Iraq and Syria.”

Jamel lived with his mother and three brothers in Hawija, a smaller town just south of the oil-rich Iraqi city of Kirkuk. He had a decent, agriculture-related job, and his family did not understand why he wanted to throw in with the insurgents who would soon become the world’s most-feared terrorist army.

In the early days, Jamel said, most of the recruits were young men in their early 20s. But soon their ranks were swollen by experienced soldiers as old as 50 from Saddam Hussein’s old army. The battle-hardened men, also Sunni Muslims alienated by the Shia government, were experienced with small arms and heavy equipment.

The mission was to take over the nation, and kill infidels and fellow Muslims who stood in their way, he said.

“Everything was about setting the role of Shariah [Islamic law],” he said. “We have to have a world based on Shariah.

“We were told that yes, people here are Muslims, but they are not the right Muslims,” he said. “And to build the Caliphate we must control the economy, take over every oil field.”

Jamel was initially permitted to carry a gun, but as ISIS grew, orders came down that only senior leadership and mid-level commanders, known as “Amirs,” could carry arms when not in battle. But Jamel would not be without his weapon long: He was made an Amir in early 2015 and put in charge of a group of 70 fighters in the heavily-contested area of Baiji.

ISIS moved on the oil-rich city of Baiji, situated on the primary road to Mosul some 130 miles north of Baghdad, a week after overrunning Iraq’s second-largest city of Mosul in June 2014. With momentum from victory, and their ranks increased by a steady stream of foreign fighters, ISIS battled Kurdish and Iraqi government forces for the city, which is home to the nation’s largest oil refinery, the Baiji oil refinery and other crucial energy and money-producing facilities.

Over the next 18 months, the city would change hands repeatedly, its residents caught in a perpetual, bloody crossfire. The prized refinery that made Baiji so critical was so heavily damaged that it may not be operational again for years.

Jamel did not offer an estimate as to how many civilians and soldiers he and his men killed, but he admitted he willfully took part in the slaughter and also handed over prisoners to his ISIS superiors for torture and execution.

Now that he is facing trial at the hands of his enemies, Jamel carefully treads the line between repentance and resignation. He told FoxNews.com he never cared for the public beheadings and civilian murders his team carried out to instill fear in conquered villages. He simply obeyed orders, he claimed, even ones handed down from the shadowy, self-professed Islamic State Caliph Abu Bakr al-Baghdadi.

“I got many orders from him,” Jamel said. “But he is a liar, he lied to us. His plan is all wrong.”

Jamel was arrested in May in a village near Kirkuk. Police in the government-controlled city had intelligence reports on him and arrested him and several associates. It was after the arrest, and as he faced justice in the Kurdish-run courts that Jamel’s conscience seems to have awakened.

“It haunts me that I am responsible for killing many people, we killed them for nothing,” he said.

Kirkuk Police Brig. Gen. Sarhad Qader Mohammad told FoxNews.com Jamel told authorities after his arrest that he had been planning to escape ISIS, a claim Mohammad said police hear nearly every time they capture a jihadist.

“When they are arrested they try to say they are no longer with ISIS, but most of the time it is not true,” Mohammad said. “And we know they are terrorists, but we have to complete a special investigation.”

This process typically takes a couple of months, during which the prisoner is held in isolation as police investigators and Kurdish security agents known as “the Asayish” interrogate them. Mohammed insisted that all captives are treated as “human beings” and are not subject to torture and other violations of international law.

Once this is complete, the prisoner is relocated and able to mix with other criminals and ISIS fighters. He or she also then faces court trial and sentencing – typically life behind bars, but sometimes a death sentence by hanging for acts of terror.

The Kirkuk Police Department currently has some 70 ISIS members awaiting trial. Since the militant group’s onslaught began just more than two years ago, some 60 fighters have been sentenced to death. Most are local men, but some are foreign fighters, Mohammed said.

“We have many operations and ways to arrest them,” he added. “We arrest them sometimes when they are sleeping, and sometimes we arrest terrorist men trying to hide by dressing as women.”