That's not what he means, or maybe he does but I didn't see it that way. A cop or a former field military lives in violence day in and day out. Sometimes they even try to cope with substance abuse. Imagine getting in an argument with somebody like that ; I've been there, thankfully he was family and I don't think could hurt me but still, I've been thinking about that for weeks and lost a decent amount of sleep.
It's unfortunately the case with tough district/city people with criminal record. Once you live in violence for more than a couple weeks, and have to assume physical threat and retaliate, I'm pretty sure it changes you somewhat fundamentally, and unfortunately those people rarely have the proper support to adjust to civil duties.