Darkling Star Trek: Voyager : Season 3 Episode 18

Stardate: 50693.2. When the doctor tries to improve his personality by incorporating the psychological profiles of famous and historical people, he is overwhelmed by their dark sides and begins exhibiting signs of a split personality, one the regular doctor, and the other dark and evil. Kes asks to leave Voyager when they encounter a race with vast knowledge of the region of space they are currently exploring.