Player Killer is a player that kills or tries to kill other players.
Refer to Player Killing for further information.
On PvP worlds player killers are marked with a skull. See further information of the Skull System.
On Hardcore PvP worlds player killers aren't tagged with a skull.
On Non-PvP worlds people can't kill another player, unless they are fighting on a Guild War.
PK is the abbreviation for 'Player Killer'. Usually Player Killers have a reason for killing other players, some of these reasons are: for items, for fun (normally this occurs in Hardcore PvP worlds), or Power Abusing.