If you’re looking for a roommate and someone writes murderer under “occupation,” you should definitely keep looking. A murderer is a person who kills deliberately and without justification.
In some instances, killing a person is considered a just or valid action — in a war, a soldier who kills another soldier is not considered to be a murderer, and people who kill in self-defense, to save themselves, are also not murderers. Killing out of anger or for money or revenge is murder, and anyone who does it is a murderer. Charles Manson is in jail for being a murderer. The Old English root of murderer is morðor, "unlawful killing."