Player Killing (プレイヤー・キル(殺人), Pureiyā Kiru?, abbreviated as PK), is the act of killing another player in an online game. With each story, the act holds different meanings and weight.
In Sword Art Online (SAO), the act of killing another player is equivalent to killing another person in real life due to the fact that Kayaba Akihiko programmed the NerveGear to fry the brain of anyone who died in SAO. Performing a crime such as hurting a green player would temporarily turn the «Color Cursor» orange. Players who had repeatedly killed others in SAO were labeled as red players by the community and, consequently, PK guilds were also labeled as red guilds to distinguish them from 'normal' criminal orange players and guilds. However, if a person attacks anyone who already had an orange cursor, the former player's own cursor will not change, since the game interprets it as defending someone or protecting themselves, just as Kirito did when he protected Asuna by killing Kuradeel. Known PK guilds are Titan's Hand and, more notably, Laughing Coffin.
Cursor color automatically restores over time for the first three crimes committed. The fourth time requires a player to complete a quest. Committing crimes five times will permanently make a player orange. Player Killers are granted a permanent orange cursor color upon their first regular PK. Though the "red" term was coined, a red cursor is only for monsters.
In order to prevent one's cursor color from turning orange, sometimes players would get other people to do the dirty work to remain green themselves, such as how Grimlock enlisted the aid of Laughing Coffin to kill Griselda and later enlisted them again to kill Caynz and Yolko. Additionally, not all members of an orange guild actually had orange cursors. An example of this is Titan's Hand, which had at least two members who remained green to scout for targets and spy in-town, while the remaining members would await at set ambush points to rob the players.
Players in Sword Art Online killed others for numerous reasons, such as enjoying killing, or believing that killing was allowed by the game, thus it did not break any game rules. Some of the players even refused to believe that people actually died when their Hit Points (HP) reached zero, hence they did not refrain from killing.
The act of killing is encouraged in ALfheim Online. The game even grants monetary bonuses to a race that manages to kill the leader of another race, such as how the Salamanders once managed to kill the leader of the Sylphs before Sakuya was elected as the leader of the Sylphs.
Unlike Sword Art Online, a death in ALfheim Online will not incur death in reality. Instead, players leave behind a Remain Light before respawning at their territory or saved spawn point after approximately a minute in the field or 600 seconds (ten minutes) in a boss room.
Players are encouraged to kill others in Gun Gale Online. Although players do not die in real life if they are killed in-game, a few members of Laughing Coffin devised the Death Gun plot to create the illusion that it was possible to murder people from within a game. To achieve this, they had one person in the game shoot another player, while at the same time another person in the real world sneaked into the player's house and used a drug to kill them in real life.
- Regular PK - Attacking a person outside the safe area.
- Portal PK - Sending players to dungeons greatly beyond their level to die.
- Monster PK - Luring hostile monsters to kill players.
- Sleep PK - Manipulating a sleeping person by moving their hand to accept a Duel and then killing them.
- Provocation PK - Manipulating and guiding players into killing each other.