Gabriel isn't exactly the best person to help someone find their self-esteem or get over past traumas. As it is, he thinks he's probably in a mental ward himself. Even if he'd never thought about it before, it seems like the sort of place he'd be if things went...poorly.

So this guy had probably lost it a little (or maybe a lot, who knows?). The fact that the same probably held true for Gabriel goes ignored. He just asks bluntly- "Have you killed anyone?"