Superboy-Prime

"I'll kill you! I'll kill you to Death!"

- Superboy Prime to Monarch.

Superboy-Prime was one of the many variations of Superman in the DC universe but became unhinged following the destruction of his world and become one of the main villains from the Crisis on Infinite Earths storyline: he had even more power than current Superman and was comparable to the Silver Age Superman in scale of power - making him easily one of the most powerful supervillains in DC comics.

Crisis on Infinite Earths
The Crisis on Infinite Earths, a multiversal catastrophe in the form of a wave of anti-matter, consumed parallel dimensions by the Anti-Monitor. Superboy aided hundreds of worlds during the crisis, but was ultimately unable to prevent the destruction of the Earth from his own dimension. Superboy later took part in the battle against the Anti-Matter in the Antimatter Universe. After the defeat of the Anti-Matter, Superboy and the others escaped to a Paradise Dimension offered to them by Alexander Luthor, Jr.

Superboy-Prime secluded himself from others in the Paradise Dimension, opting to replay events from his life on Earth Prime using crystals. Over time, Superboy-Prime becomes frustrated, and attempts to reach out to Kal-L. Superboy-Prime was then persuaded by Alexander, who had come to believe that the Paradise Dimension was the reason for Lois Lane's failing health, to help him return to reality. When Superboy-Prime hesitated, Alexander showed him the negative aspects of the Post-Crisis Earth, including the deaths of his parents and girlfriend in a car accident.

Altering Reality
Superboy-Prime, infuriated, shattered the barrier of reality. This assault resulted in shifts in DC continuity, including; the resurrection of Jason Todd; the combining of Superman's origins; the Doom Patrol's rebooting as new characters; Donna Troy's mutliple origins; the various incarnations of Hawkman; the different incarnations of the League of Super-Heroes.

Countdown to Infinite Crisis
Superboy-Prime join Alexander in his quest to bring with his Anti-Matter powers using it to break free. He was spotted by Blue Beetle, but Prime ignored him. In quick succession, Superboy-Prime obtained the Heart of Darkness diamond to be obtained by Jean Loring, altered the location of Thanagar, initiating the Rann-Thanagar War and attacking and capturing J'onn J'onzz, destroying the Watchtower.

Personality
Originally destined to be the greatest superhero and the world's chosen champion in the fight against evil, Superboy-Prime, left scarred by the traumatic events he witnessed has become a brutal killing machine. Years of isolation in the paradise dimension while endlessly viewing the events of a life he could no longer have put severe stress on his already fragile mind. During Infinite Crisis when the turning point came and he accidentally killed several heroes his mind finally snapped and he retreated into denial of the horrific nature of his actions. After being further imprisoned by the Flashes and spending three more relative years imprisoned again, Prime has buried any remorse or compassion under an insane belief that since he is a Superman, everything he does is right and views any other reality apart from his own as an imperfect duplicate of his world. All of this has left Prime in a psychotic state of mind unable to distinguish the morality of his own actions and the ability to commit even the most heinous of deeds with no guilt at all. Hellbent on nothing but killing numerous superheroes while also leaving an endless wave of utter destruction in his wake and as such, is one of the most dangerous villains in the universe.

Powers
Superboy-Prime has had a collection of powers over time, being one of the many alternate Supermen of DC comics he possesses standard Kryptonian powers but was able to amplify them via power-armor and other devices: over the course of the Crisis Superboy-Prime became more and more powerful, at the peak of his power he had absolute control over time to the point he could create pocket universes.

Also due to Superboy-Prime being from another universe he was immune to most of Superman's weaknesses, such as magic and kryptonite - his power-suit was also designed to constantly feed him yellow sun radiation and keep him at peak power even when exposed to red sun radiation.