He’s very mild mannered for a Necromancer and sometimes overly polite. While he has been turned to a Vampire, he uses the powers given unto him to enhance is amazing abilities.
Theodas isn’t afraid to commit a murder. But he doesn’t make a point of it other than to feed for his bloodlust Vampirism. Or if there is a contract from the Dark Brotherhood or if a job of the Thieves Guild goes south. Otherwise he’s an adventurer.