Evil

From Memory Alpha, the free Star Trek reference.

Adolf Hitler is often seen as a contemporary portrait of Evil because of his actions during World War II

Evil is an abstract concept that incorporates a set of religious, or moral ethics antithetical to Good. Evil is usually destructive, malignant, and chaotic. Evil can be used to describe actions or individuals (even non-corporeal entities), as well as ideas.

The Excalbians were interested in learning about the difference between Good and Evil when they pitted Kirk, Spock, Surak and Abraham Lincoln against Genghis Khan, Colonel Phillip Green, Kahless and Zora. (TOS: "The Savage Curtain")