The original Kingmaker was Richard Neville, 16th Earl of Warwick. He was instrumental during the Wars of the Roses in the deposition of two kings. The term kingmaker has come to mean, "a person or group that has great influence in a royal or political succession, without being a viable candidate." The term in game theory has come to mean:
Kingmaking: in a game of three or more players (on three or more opposing sides), during and endgame situation when a player who is unable to win is able to determine which player among others is the winner.
Kingmaking to me basically comes down to purposefully taking actions in a game so that someone other than yourself wins. It is slightly unusual that only the actions of the last player are considered kingmaking, when it is the total contribution of all players over the entire course of the game that determines a winner (i.e. If some else prevent you from getting 4 VP from shipping on boats in Puerto Rico on turn 5, but I prevent you from getting 4 VP on turn 19 and you lose by four points...why am I the kingmaker?)
Any game with more than two opposing sides is susceptible to kingmaking. In the world of competitive gaming, kingmaking can even occur in games with only two sides (due to rankings of multiple players). In Sumo Wrestling, for example yaochō or "match fixing" is where one wrestler who has already achieved a high paying rank throws a match on purpose to allow the opposing wrestler to secure a higher paycheck. In Magic: the Gathering tournaments, where a player may have no chance of making the top 8, they might concede a match to help a friend have a better win-loss ratio, while trying their hardest against any other opponents.
In a multiplayer MtG game, players may openly collude to help destroy the player with the best board position. In other groups, tabletalk and open collusion banned, but it is still expected that most players will gang up on the player with the best board position. The rules for MtG can also be exploited in multiplayer. A kingmaker player on the verge of elimination might cast an instant to hurt an unfavored player in hopes that they don't win or to make the game end earlier so they can play another game. A kingmaker might concede before damage is dealt so an unfavored player doesn't gain life from lifelinked creatures.
There are other examples from games like Ticket to Ride - purely blocking moves, and Settlers of Catan monopoly card ethics and future trades (these aren't the exact links I was looking for. I thought that one of these talked about the ethics of performing multiple trades circumvent the no zero card trade rule) , Monopoly trade before losing. Looking over the highest scored answers, it appears that there is still some question about whether kingmaking is a problem or the normal part of any game. In Risk Legacy, the line is blurred even further since the game is played over multiple rounds. A player without the chance of winning might purposefully lose to a particular player to improve their odds in future games.
Is kingmaking a problem that needs to be fixed? (Are only certain kinds of kingmaking an issue? Collusion, playing to lose/disrupt the game, etc.)
What steps/rules are helpful in mitigating kingmaking in games? (general concepts, but real world examples preferred)
to deliberatly play an action as player A, so that player B wins instead of player C.