A skullhunter, also called skullbasher or pkk (playerkiller killer), is a player that tries to kill every Playerkiller they see. Most do it because they won't get a skull for the kill, or because they just want some quick Money or Equipment. Some people kill skulled players as a sort of vigilante justice, to further punish them for killing or attacking another player.
While some people find it rewarding, skull hunting is a dangerous profession, as you never know what kind of equipment, skills, abilities, and friends the other player has. Many times, a skullhunter is killed by their prey and/or their prey's companions (especially if that player is the member of a Guild).
Another drawback to skull hunting, is that you may be hunted in return, either by the person you killed, or by their friends. It is also possible that a player has a higher-level character in that world, and they may send their other character to seek vengence upon you.
In a Non-PvP world, Skull hunting does not directly apply, since it is not possible to attack another player directly, and therefore, it is not possible to get a skull . Similarly, in a PvP-Enforced world, the Skull System is not in place, and therefore, there cannot be true skullhunters. In both of these situations, you may still be hunted for various reasons, and players employ different tactics for punishing thieves, Playerkillers, and Trap-player killers.