"The Man He Killed" isn't your average war poem. It's personal, brief, and haunting. There are no grand generalizations, no waxing poetic about glory and sacrifice, and no grisly, graphic battlefield scenes. It's just a guy, telling someone about the fact that one time, he shot a man down. It's as simple as that. Except that it isn't.
This is not an anti-war poem. It's merely pointing out a fact of war—that men have to kill each other for reasons they don't understand.
This is totally an anti-war poem, because Hardy is pointing out the senselessness of these deaths. Men who could have just as well have been friends should not be killing each other for the sake of some cause. They're not really foes in the first place.