His power is actually a string of fate with Saitama. Every monster that "killed" by King is actually killed by Saitama, every single one of them, even before King is recruited as hero. In fact, the scar in King's face is resulted from monster who later killed by Saitama, but everyone near him think King killed the monster. Another fun fact, in the first chapter of this manga, where Saitama kill the vaccine man, King is nearby and later credited for killing that monster.