At first, the poem seems to describe a typical bully: someone who says cruel things, hurts people, and feels regret afterward. He plants "a bad seed" even when he tries to be kind, and the pain keeps growing. The insults, "useless, stupid, piece of crap," sound harsh and automatic, as if he has said them so often that he no longer notices.
The last stanza changes everything. His words were never really aimed at other people. They were aimed at himself, at the mirror. The bullying we thought we were watching turns out to be self-hatred, and the speaker's quiet, matter-of-fact tone makes that reveal sadder.
The mood is heavy and sorrowful, with a thread of sympathy running through it. The poem doesn't excuse cruelty, but it suggests that the person who hurts others may be hurting most of all inside, and that nobody around him can see it.