In the Greek pantheon, Hades rules over the Underworld. At one point, however, he's developed higher aspirations - taking the throne of Zeus! To do this, he attempted to get rid of Hercules, Zeus's demigod son, the one person that stood between him and domination.
Of course, he'll cool off if he has to, or if things are too much for him to handle. He does have a responsibility with the dead, after all.