Here is an interesting essay by Georges Dreyfus from 2011 about the concept of “evil” in Buddhism, specifically in Tibetan Buddhism. Dreyfus examines in this essay among others the concept of “evil” in the context of the deity Dorje Shugden / Dolgyal. The essay highlights also the classes and functions of Dharma protectors, and the traditional role of Shugden in the Gelug school.
I think it is an important essay to get a better understanding about these tricky issues:
The Predicament of Evil: The Case of Dorje Shukden by Georges Dreyfus