Star Trek: Voyager: Season 3, Episode 18

3 - 18: The Darkling

Stardate: 50693.2 When the doctor tries to enhance 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 considers leaving Voyager when they encounter a race with vast knowledge of the region of space they are currently exploring.