Star Trek: Khan Ruling in Hell

When the most infamous villain in STAR TREK history is last seen in the Original Series episode "Space Seed," Khan Noonian Singh's an elegant, proud warrior-king. When he's next seen in STAR TREK II, he's a grizzled maniac, twisted by loss and an unquenchable thirst for revenge. What has happened between these two points in time to so radically alter what seemed to be an unbreakable will?