The speaker is asking a painful question: is it wrong, even monstrous, not to feel what a brother should feel for someone? The "box" is the poem's strangest image. It could be a coffin, which would make this a poem of grief and survivor's guilt, but the poem never says so. It might also be something else that confines or hides a person, so the reading stays open.
What comes through clearly is the speaker's unease. Calling himself a "villain" shows how harshly he judges himself, and the question mark he never quite writes is carried by the phrasing itself. In the second part, ordinary pleasures have lost their sweetness, and he apologizes for that. The apology is odd and moving: he is sorry not only for failing to feel enough, but for the enjoyment he can no longer have.
The mood is quiet, bare, and uncomfortable. Short lines and plain words leave little room for comfort, so the guilt sits there unresolved.