Insanity

Insanity was a condition of mental instability in sentient beings. It was colloquially referred to as madness or going crazy.

The Master is a prime example of insanity. He was driven crazy by the constant sound of drumming in his head, due to Rassilon's plan to escape the Time War/Time Lock by using it as link. (TV: The End of Time) As Missy, the Master had openly claimed to be "bananas"; unlike her past lives, Missy mostly forwent using strategy solely for the purpose of amusing herself, and to annoy the Doctor. (TV: Dark Water/Death in Heaven)

The supercomputer Xoanon was inadvertently driven insane when the Fourth Doctor helped repair it. (TV: The Face of Evil)

A psychic locket showed Alex Hopkins the future, and also drove him insane. (TV: Fragments)

Dalek Caan became insane after he saw time itself. (TV: The Stolen Earth/Journey's End)

Giuseppe di Cattivo was driven insane after painting The Abomination. (TV: Mona Lisa's Revenge)