SYRIA -- He's treated like a dangerous criminal. And he was trained to kill by ISIS, before being captured by Kurdish soldiers.

But Mohammed is also a frightened 19-year-old. He asked us to hide his face for his mother's sake.

"She often told me to leave ISIS," he said, "but I never obeyed her."

Mohammed is walked into a room to speak with CBS News' Holly Williams CBS News

He grew up in a Muslim family in Syria, but told us he knew very little about Islam until he was recruited by an uncle and a village elder.

"They recited verses from the Koran to explain that Muslims must fight," he said. "Then they sent me to a camp to learn about Islamic law, and gradually I became convinced."

Mohammed seems less a committed extremist, than simply naïve. It doesn't lessen his crimes, but it shows that ISIS -- which relies on fighters who will die for the cause -- has a weakness.

Mohammed told us he began to lose faith in ISIS when he witnessed one of the group's many brutal beheadings. He said he did not think that was the real Islam.

"It was a horrific. I wish I'd never seen it."

He also told us that U.S. coalition airstrikes are taking a heavy toll on ISIS. He and other fighters recently had their food allowance cut.

"They told us the airstrikes were hitting their oil installations and they weren't making as much money as before."

What Mohammed said was confirmed by pretty much everyone CBS News spoke to who is fighting ISIS in Iraq and Syria. The have fewer fighters, they're losing territory, and they also seem to have less cash.