@mahtan Didn't you hear the bandit leader? They're worth more alive than dead.
In all seriousness, though, it's likely that trite "if you kill them, you're no better than they are" bullshit that you see across every single culture. Yeah, sure, if I kill a killer, the number of killers remains the same, but if I kill twenty killers, I think we're doing pretty good on the whole killer/not-killer scale.