A new mutant's powers manifest. His powers are energy creation, manipulation and control on a cosmic Franklin Richards scale. When they first manifested he blew up his hometown but it wasn't just a simple explosion. To everyone outside the destructive wave or bubble of the explosion it was an instant, but everyone inside had to experience their entire body slowly being vaporized, through whatever sort of science jargon technobabble is used to explain the rate at which it happened to those inside. The Mutant who's powers did this is stricken with grief over what he did. The Krakoan mutants try to help him but keep throwing their "being a mutant is really good". He keeps telling them he just wants to be left alone and he doesn't want these powers. He should be locked away or killed so he can't hurt anyone with them. Xavier and Co. keep trying to instill their Mutant indoctrination partially out of truly helping him, but mostly because he's to powerful to not have on their side.
He rejects the mutants and tells them to leave him alone. They don't listen and continue to show up and interfere with his life. He learns about a possible cure or way to dampen or shut off his powers so he can live a normal life but the X-Men show up and destroy the tech as it is a threat to Mutants and then go on about how important he is and that they need his powers if Mutants are to ever stop being oppressed by humans. This pushes him over the edge and he uses his powers for the first time since their creation and fights the X-Men, eventually going full Nitro and just decimating everything and killing them.
From this point onward he shows up on Krakoa and just lays waste to the place, unable to be stopped because his powers are on a cosmic level and just because I hate them, his powers also shield him from telepathic manipulation. After ravaging the island he has Xavier at his mercy, but lets him live, because I really just want someone to push their shit in for their hubris