I hope you had a good Halloween. For the occasion, here is a Buddhist legend, particularly associated with Vajrayana or Tibetan Buddhism. Once there was a man who heard Buddha preach. So mired in delusion, craving, and aggression was he that he interpreted Buddha's words as a license for indulging himself to the fullest. When the Wheel of Life rolled around, he was reincarnated as a terrible monster, Rudra, and gained great power in all the hells and all the earth.
Faced with such an unregenerate egoism, the buddhas knew that they could not turn Rudra to compassion by appearing to him in their usual form. Therefore they manifested themselves as the herukas. The herukas, also called the wrathful deities, are expressions of buddha-consciousness and buddha-compassion, but under a terrible aspect. They are the expressions of wrathful compassion. Their visible representations are terrible monsters that drink blood.
The way of it is this. Sometimes people so identify themselves with their delusions, cravings, and aggressive impulses that the compassion of the buddhas is something threatening. The buddhas wish to destroy our attachment to this world, to break our slavery to aggressive impulses, to snap the bonds of our delusions. When we identify ourselves too fully with these things, it's not surprising that buddha-compassion has a terrible aspect: it destroys, crushes, devours the very things we think we are. It does so in order to free us, of course; the blood-drinking buddhas have clear minds and compassionate hearts. But their clarity of mind seems to the soul mired in egoism to be terrible and violent (as truth sometimes seems to be), and their compassion of heart appears to be something that devours and destroys (for they devour and destroy our attachments to this world).
In this way the buddhas were able to subjugate the terrible monster Rudra, bringing him to enlightenment and compassion.