"Kingmaker" is an expression used in gaming
in general (not necessarily computer games, it is in fact more common among players of board game
) to describe the role of a player who, although he has no chance of winning anymore, is able to influence or even outright decidce which of the other players will win. Obviously, this can only occur when there are more than two players.
An typical example would be this: a Monopoly player who is losing but not yet bankrupt may trade properties to another player which that player needs to complete monopolies, not because the trades are in any way beneficial to the losing player, but simply because he wants to help the other player against a third (probably leading) player.
A potential for kingmaking is generally considered an undesirable quality in games because it is frustrating to lose a game you were winning by skill to someone holding a grudge. Worse yet, it can be used as a threat by spoilsports: "Attack me now, and I'll make sure you don't win, even if it means I lose everything!".
Of course this can be discouraged by social means, and in fact kingmaking can be seen as a somewhat positive aspect of the meta game that punishes excessively competitive behaviour and gloating. Nevertheless, game designers strive to avoid it, and reviewers rightfully consider a strong kingmaking potential a negative point.