- "Spock, I've found that evil usually triumphs unless good is very, very careful."
Evil was an abstract concept that incorporated a set of religious, or moral ethics antithetical to Good. Evil was usually destructive, malignant, and chaotic. Evil could be used to describe actions or individuals (even non-corporeal entities), as well as ideas.
The sacred book worshiped by the Yangs of Omega IV promised that good was stronger than evil – that good must destroy evil. As such, the Yangs feared the return of evil's embodiment, known as the Evil One. (TOS: "The Omega Glory")
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. The Excalbians later concluded the Evil retreats when confronted but that Good and Evil forces tend to use the same methods and achieve the same results. (TOS: "The Savage Curtain")