Player Killing, often abbreviated as PK, is the act of killing other players in an online game. With each story, the act holds different meanings and weight.
In SAO, the act of killing another player is equivalent to killing another person in real life due to the fact that the NerveGear would fry their brains. The most pronounced case of murderers was a PK guild called Laughing Coffin, a red guild. Performing a crime such as stealing, hurting, and killing green players would turn the «Color Cursor» orange. Players who had 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 won't change, since the game interprets it as defending someone or protecting himself/herself, just as Kirito did when he protected Asuna by killing Kuradeel.
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 permanent orange cursor color.
In order to avoid one's cursor color being turned orange or red, sometimes players or members of orange guilds would send other people to do their jobs for them. This way their cursors could remain green, and they could disguise themselves amongst other players.
Players in SAO have player-killed for numerous reasons. Some said reasons being that they enjoyed killing, or that they felt it was okay since there was no punishment, and some of the players refused to believe that people actually died when their HP reached zero, hence they did not refrain from killing other people.
The act of killing was encouraged since it was territorial battles which resulted in obtaining items as well as other benefits such as territory and power. Most notable is the Salamander struggle against the Sylph which is brought up numerous times in Fairy Dance.
Unlike Sword Art Online, a death in ALfheim online will not incur a death in reality; instead, players leave behind a Remain Light before respawning at their territory or saved spawn point, after approximately 600 seconds (10 minutes).
Players are encouraged in killing others since the guns which were effective on monsters were not effective on humans, but live ammunition guns worked well so a separation on gun choice determined play style.
However, during Gun Gale Online, Kirito encounters an incident where PKers from Laughing Coffin come back and take advantage of a human's immobility while playing a MMORPG. They created an illusion where one ex-Laughing Coffin member would be in the game and as he shot another player, another ex-Laughing Coffin member in the real world sneaked into that player's house and killed them in real life.
- Regular PK - attacking a person outside the safe area.
- Portal PK - Sending players to dungeons way above their level to die.
- Monster PK - Using neutral monsters to kill players.
- Sleep PK - manipulating a sleeping person by moving his/her hand to accept a Duel and then killing them.