Griefer

A griefer is a player in a multiplayer video game, such as Minecraft, that purposely harasses other players in the game. The term is similar to a troll or griefer in a forum or video game.

A griefer doesn't intend to actually play the game but just to annoy other players. He or she uses game aspects in unintended ways, which includes stealing from, killing, and destroying other players.