The Kryptonite Man
There have been three separate individuals that Superman has encountered in the course of his adventures who have used the name The Kryptonite Man.
The first of these individuals is a disguised Lex Luthor (Act No. 249, Feb 1959: "The Kryptonite Man!").
The second Kryptonite Man begins his career as the Kryptonite Kid and later appears in the chronicles as an adult (S No. 299, May 1976: "The Double-or-Nothing Life of Superman!").
The third being to use the name the Kryptonite Man had been the ruler of a race of humanoids who inhabited Krypton eons before Superman's ancestors. When a nearby cosmic body threatened life on Krypton, the Kryptonite Man, whose real name is never given, sends all of his people into stasis deep underground, while he himself remains on the highest mountain peek, to act as their guardian. He is then placed in suspended animation for what is to be 20 years, at which time, he will awaken to determine if the planet is habitable once again. For some unknown reason, the machinery fails, and the unnamed ruler slept for over a thousand years. Unfortunately, the very day he awakens, is the day Krypton explodes.
The mountain that the unnamed ruler's observatory is located on is sent into space, and somehow, the king is able to feed off of the kryptonite radiation produced by this event. He eventually becomes dependent upon these energies to sustain his life, and becomes weak outside of its influence.
This Kryptonite Man blames the human inhabitants (Superman's race) for the death of Krypton, never realizing that it was a natural disaster. Eventually, after the kryptonite radiation of the mountain begins to fade, the Kryptonite Man encounters a race known as the Seeders. For unknown reasons, the ships of this race produce radiation similar enough to kryptonite radiation that the Kryptonite Man is able to survive.
The Kryptonite Man discovers the existence of human Kryptonians on Earth, and stealing a Seeder ship, travels there to confront, and kill them. He attacks Superman and Supergirl, but their battle is interrupted by the Seeders, who take offense to the Kryptonite Man's theft of their spacecraft. Eventually, the Kryptonite Man sacrifices himself to prevent the Seeders from destroying the Earth in their zeal to obliterate him, killing the Seeders along with him (S No. 397, Jul 1984: "The Born-Again Kryptonite Man!"; SG2 No. 21, Jul 1984: "Curse of the Kryptonite-Man!").