The poem opens with a strange, almost dreamlike image: a horse's head caught in a tree. The speaker says it showed them "the whole mystery of abuse." We aren't told what the head means, and that gap is part of the effect. It feels like a sudden, unsettling glimpse of how cruelty works, without any neat explanation.
The second part turns to broader reflection. "We understand backward" suggests that we only make sense of harm after the fact, when it's too late. The lines about "dying more of envy than true love" and killing other people hint that jealousy and resentment drive more damage than affection ever does. Then comes the odd, cold ending: "but let the rules be." It could mean people keep killing while insisting the rules stay intact, or it could be a weary shrug at that hypocrisy.
The mood is dark, puzzled, and quietly bitter. The short lines and abrupt shifts leave the reader a little off balance, as if the speaker is still working out what they saw.