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) and explained to a glass avatar of Bill Potts that it was one of the reasons he stole a TARDIS and ran from Gallifrey.
There is good and there is evil. I left Gallifrey to answer a question of my own. By any analysis, evil should always win. Good is not a practical survival strategy. It requires loyalty, self-sacrifice, and love. So, why does good prevail? What keeps the balance between good and evil in this appalling universe? Is there some kind of logic? Some mysterious force?
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)
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)