The speaker of "The Man He Killed" is a man who is reflecting upon having to kill another man in battle. We learn that he didn't kill a man because he hated him. In fact, he is sure that if the two had met in a bar at another time, he would have treated the man to drinks.

The speaker thus reflects on the necessities and senselessness of war. Although two men have no personal grievances toward each other, they are thrown together by circumstance and forced to kill—or be killed. He has shot this man because "he was [a] foe." And he was only a foe because the causes they fought for established the lines of friendship and enemies.

We also learn that the speaker didn't join this war because he felt passionately about the causes it espoused. Instead, he simply joined because he was out of work and had already sold all his belongings. He is mindful that the man he's been forced to kill after coming "face to face" with him could have found himself in the same position, joining a war simply to avoid starvation and destitution.

There is no clear indication of setting, but we do know that the entire poem is written in quotation marks, so it seems to be some sort of conversation. Combined with the casual phrasing and long pauses, even within lines, it's easy to imagine that the speaker is conveying his thoughts at one of those local pubs he references in the poem.