From SA Mafia Wiki
A Nightkill is the act of killing another player at night in a game of Mafia. Nightkills are typically performed by scum such as Mafia members or Serial Killers. The nightkill is the Mafia's main method of fighting back against the Town, as it can be used to silence power roles, skilled players, and confirmed townies. Nightkills are very important night actions to any game of Mafia, as they provide concrete information on a person's alignment and opinions via their cardflip, letting the town draw connections (and the scum create false ones).
Nightkills can be prevented in a number of ways. If a nightkill is directed at a protected or bulletproof player, the kill will fail. The kill can also fail if the person who carries out the kill is roleblocked. Despite the many different ways of preventing nightkills, it is a common assumption among players that nightkills are prevented solely because of skilled Doctors, when in reality a number of factors could have prevented the kill. Roleblockers also occasionally hastily connect their night action to the lack of a kill, which can lead to the wrong person being lynched.
Nightkills are typically accompanied by flavor, although it is the moderator's preference as to whether the flavor remains consistent between players/groups from night to night.
While scum comprise the majority of nightkills, some town roles such as vigilantes may kill at night, and other roles such as Bombs, Paranoid Gun Owners, Bodyguards, and Surgeons can cause night deaths.