One of the things I like about this universe is that core characters seem seriously angry at being forced to kill people--angry at the people they kill. People lose control of their emotions. The best example of this was when Thor, after all his entreaties and appeals to the Ultimates, finally just lost it and started zapping the hell out of everybody. And the Widow grinding Thor's head into the ground and preaching maniacally after she sucker-punched him.
It's real-world anger: Somebody has forced you to lower yourself into a fight, and your anger explodes once you've beat them down. It's sort of an enraged regret. It's identifiable. Compare this to, say, the MU Wolverine #27, where he casually counts up the number of people he intends to kill for turning him into a Hydra-bot. Fifty-two thousand? Hey, no problem.