"Killjoy" is a short, bleak self-portrait. The speaker begins by calling himself "a missing man" and "a captain without a voyage": someone absent even from his own life, with a role but nothing to command. There is a sense of purpose gone idle.
The next lines are harder to pin down. He says he opposes wonder, which is "so old a hush," as if awe is something ancient and quiet that he wants to push against. The title fits here: he is the one who spoils delight. Yet he also offers to fell all the winter's timber, a hard, practical job. Work seems to be the only way he knows to be useful.
The ending is the strongest part. He declares his need for "ice / and wind and pain." The speaker doesn't want comfort. Judging by the imagery, he wants something harsh enough to make him feel present again. The mood is cold, stripped-down, and quietly desperate.