By Hollywood standards, "The Man He Killed" isn't that violent, and that is the real power of the poem. While the speaker entertains the idea of meeting the other man in a different context, he doesn't think once about what it would mean to kill another person in any other context than war. In fact, he doesn't really think much about what it means to have killed someone in war either. We're never even sure that the speaker thinks of the event as violent in the first place. It sounds like just another day on the battlefield, which is exactly the point.
Lines 7-8: This is about as unemotional as one can get while talking about the fact that you've shot someone who was standing right in front of you. Usually this lack of emotion is found in psychopaths, but we guess it's found in warriors, too.
Line 12: The speaker might not intend this, but this line is totally ironic. The speaker says that his reason for killing the man is clear enough, but that's not really true at all. Even he doesn't really understand why he shot this guy.
Line 17: Poets through the ages have written about war, but no one ever called it quaint or curious, at least, not that Shmoop remembers. These adjectives undercut the violence for our speaker, but in a weird way, they also highlight the violence for us readers. Once we hear war described as "quaint and curious," we can't help but immediately think of how it's really just not.