PK'ing, or Player Killing, is an unfortunate (though sometimes wanted) feature of many adventuring MUDs. It is simple what happens when one player has their character kill the character of another player, as opposed to some computer controlled NPC. Interestingly this very seldom happens in real RPG's, probably because the social atmosphere of an RPG makes the players realise that the other players are real people, people with whom they are co-operating to make a good game. Online it is easy to forget the player behind the character, thus you don't think about them, only the thrill of killing another creature.