Havoc is about a speaker who feels broken from the start and is caught in a painful contradiction. They describe a "monster" inside them that is ruining them, but they can't simply fight it, because it lives within them. Attacking it would mean hurting themselves too, which is the meaning of the line about "burning along."
What makes the poem unsettling is that the monster is also "beautiful." It gives the speaker the only feeling they've had "for a long while," so destroying it would mean losing the one thing that makes them feel alive. The poem doesn't say what the monster is, and that openness lets readers fill it in with their own idea of a damaging habit, emotion, or desire.
The mood is raw and conflicted. Short lines and repeated questions ("Why should I?" "Why would I??") sound like someone arguing with themselves, drifting from despair toward a troubling kind of acceptance.