Good was the concept or force perceived as being the opposite of what was evil, both of them being part of moral duality. However good, like evil, could be perceived differently with conflicting views, as there were individuals who considered the Doctor's good to be their evil, including Sutekh (TV: Pyramids of Mars) and the Black Guardian. (TV: Mawdryn Undead)
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)
Although the Time Lords did not believe in nor gave much thought to the concepts of good and evil, the Doctor had always maintained that evil was a real force. (PROSE: Strange England, AUDIO: The Guardians of Prophecy, COMIC: A Groatsworth of Wit) The First Doctor explained to a glass avatar of Bill Potts that it was one of the reasons he stole a TARDIS and ran from Gallifrey:
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) After much deliberation, (TV: Into the Dalek) the Twelfth Doctor came to the conclusion that he was neither a good man nor a bad man, but rather "an idiot", one "with a box and a screwdriver [...] passing through, helping out, learning". (TV: Death in Heaven)
Missy considered the needless murder of a married man who had a child to be evidence enough that she had not "turned good". (TV: The Magician's Apprentice) As the Master was the only person the Twelfth Doctor had ever met who he felt was remotely like him, the Doctor wanted nothing more than for them to stand by his side and be good. (TV: World Enough and Time) He found this hope difficult to resist. (TV: The Eaters of Light) Even while "really trying not to kill anyone today" to impress the Doctor, Missy proudly expressed that being "more evil" automatically meant she was "cleverer". (TV: World Enough and Time) Missy, in the end, admitted that standing with the Doctor was all she had ever wanted, too. She ultimately died, by her past self's hand, trying to join him at last. (TV: The Doctor Falls)
Estelle Cole believed that the fairies were good, while Jack Harkness believed they were bad. This led Gwen Cooper to observe that "one person's good could be somebody else's evil", which Estelle recalled echoed the sentiment of Jack's father, actually Jack himself. (TV: Small Worlds)
After witnessing the evil of the Daleks, (TV: The Daleks) and warring against them, the Doctor hoped to one day meet a good Dalek. (AUDIO: Echoes of War; COMIC: Defender of the Daleks) When pressed by Clara Oswald on the difference that one good Dalek would make, he revealed his hope that one good Dalek meant that he could one day turn the entire race away from their perpetual war. (TV: Into the Dalek)
Facing the "Metaltron", who he believed to be the last surviving Dalek, the Ninth Doctor angrily ordered it to kill itself, thus making the Daleks extinct. In response, the "Metaltron" remarked that the Doctor "would make a good Dalek", leaving the Time Lord in stunned silence. (TV: Dalek)) The Dalek named Rusty, who saw the hatred for the Daleks within the Twelfth Doctor's mind, deemed it good and was inspired to kill the "evil" Daleks. Seeing that Rusty was motivated by hatred, the Doctor did not see him as a good Dalek and so did not see his extermination of attacking Daleks as a victory. Rusty conceded that he was not a good Dalek, instead seeing the Doctor as a good Dalek. Again, the Doctor offered no retort. (TV: Into the Dalek)