From SA Mafia Wiki
A CPR Doctor (or occasionally Surgeon) is a protective role that has the function of a doctor as well as a vigilante. If the CPR Doctor protects a player who is attacked during the night, the CPR doctor will function as a doctor and protect that player. If the CPR Doctor protects a player who is not attacked, however, he will kill that player.
It is common for CPR Doctors to be informed of their status, but in some cases may be led to believe they are a normal Doctor until their experience in the game shows them to be killing people instead of protecting them (in the same manner an insane cop may be led to believe he is a normal cop).