Tibia Wiki

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 Skull System.
On PvP-Enforced worlds player killers aren't tagged with a skull, and you also gain experience depending on the level of the player whom you kill.
On Non-PvP worlds people can't kill another player.

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 PvP-Enforced worlds), or Power Abusing.