The cigarette dangling from Khalil's lips gives him the air of teenage rebellion generally reserved for boys his age. But Khalil's rebellion is darker than most.

Khalil says he ran away from home to join ISIS a year and a half ago. Today, he sits in a rehabilitation center in Syria's Aleppo province for former extremist fighters. At the age of 14, he's the youngest in his class. As a former child soldier, Khalil is classified as a "level 2" detainee -- an active ISIS fighter.

He's shy and struggles to find words to express himself, looking away sheepishly as we ask him about what he was thinking the first time he carried a gun in battle.