Why do the deities appear in peaceful and wrathful forms? It is for the liberation of special beings. Normally, bodhicitta is effective in subduing evil beings. When Sakyamuni Buddha attained enlightenment, he overwhelmed Mara’s troops with bodhicitta; but it is only when one has garnered the same kind of power as the Buddha that bodhicitta can function effectively in this kind of situation. Bodhicitta is not useful against powerful demons that are endowed with great blessings from a previous life. As demons are also subject to causality—some due to hatred, others the ripening of past evil aspirations that make them what they are now, which have little to do with renunciation and bodhicitta, only the appearance of wrathful deities can subdue them.

- Quote from Gateway to the Vajrayana Path, "The Parable of a Black Snake"