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.
A 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 an embodiment of evil, 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, a recreation of Surak, and a recreation of Abraham Lincoln against a recreation of Genghis Khan, a recreation of Colonel Green, a recreation of Kahless the Unforgettable, and a recreation of Zora. The Excalbians later concluded that evil retreats when confronted, but also that good and evil forces tend to use the same methods and achieve the same results. (TOS: "The Savage Curtain")
"Or by misleading the innocent."
"You do not understand. I do not serve things evil. I am evil."