These monsters are representations of the enlightened mind and the attainment of Nirvana. All these beings are Buddhas who lead to enlightenment, the Mahakala represents the Will, the True Will.
Kali is represented as a bloodthirsty black giant. These "monsters" are the best helpers who elevate the monk.
Indra is also seen as a protector of the Dharma, and the Vedic gods are generally well regarded.
In general, the most powerful deities in Vedism are Buddhas for Buddhists.