Evil

Evil was the concept or force perceived as being the opposite of what was good, both of them being part of duality. However, like good, evil could be perceived differently with conflicting views; as such, the Seventh Doctor claimed evil had no name. (TV: The Curse of Fenric)

In the distant past, the Grey Man's people imposed duality onto the first humanoids to evolve, resulting in them destroying themselves in chemical warfare. The Grey Man later created Cathedral as a metacultural engine that introduced "greyness" and "doubt" into the universe. (PROSE: Falls the Shadow)

The Time Lords did not believe in nor gave much thought to the concepts of good and evil, although the Doctor had always maintained that evil was a real force. (PROSE: Strange England, AUDIO: The Guardians of Prophecy) In his travels he met many "embodiments" of evil, including the Black Guardian, (TV: The Armageddon Factor) the Mara, (TV: Snakedance) Malador, (AUDIO: The Guardians of Prophecy) and Fenric, whom the Doctor told Ace was one of two forces, only good and evil, that existed before the beginning of all time and space. (TV: The Curse of Fenric)

While the Doctor often tried to be a force for good, his methods could be seen as evil, causing other races to raise arms in fear against him, (TV: A Good Man Goes to War) with an alliance including several of his enemies imprisoning him in an attempt to prevent the end of the universe. (TV: The Pandorica Opens) Several individuals considered "the Doctor's good" to be "their evil," including Sutekh (TV: Pyramids of Mars) and the Black Guardian. (TV: Mawdryn Undead)

The Fourth Doctor called the Master the "quintessence of evil" (TV: The Deadly Assassin) while Borusa told him that he was "one of the most evil and corrupt beings [that] Time Lord race [had] ever produced." (TV: The Five Doctors)