The Hulk is many things. What, exactly, depends on the time of the story, and the necessity thereof. The Hulk used to be a monster, born of gamma radiation, Bruce Banner's repressed rage unleashed. His helplessness as a child as an abusive father beat his mother forced him to repress his anger, until it finally was able to get out as the Hulk. However, Bruce didn't just repress his rage. As his life went on, he repressed his guilt, creating yet another Hulk, deep inside his mind. And as he became more and more resentful of the world hunting him, he created a powerful monstrosity of such might it took several Hulk's to lock it away within his mind: The Devil Hulk, who wishes only to see the world burn.