Kingmaker (in the context of gaming) = player who can decide the winner of the game while having no chances of winning himself.
Ok, I realize this is a niche subject, as it encompasses rare scenario in already rare multiplayer Dominion; but I think it would be an interesting topic to discuss, as it touches upon fair play, etiquette and ever-loved edge cases.
I firstly encountered it years ago, while still unfamiliar with the term and while playing a different game. It was a 4 player Ad Astra game where, in the end, only leading two players had the chance of victory, while myself and 4th player were way far behind. So, in the last round where everybody was prepared to cash in points, I played in a way that completely screwed up one of leading players and enabled his rival an easy victory. It was not my intention, of course - I simply played in a way which would maximize my own score (regardless of having zero chances of winning), while messing up with other guy's plan was just a side-effect. After the game we had a discussion of whether this was a fair play from me or not.
I found myself on the other side of this case a while ago, in a 3P Dominion game. Basically, Player B was much ahead of me, but my deck was quickly catching up. Problem was, Province pile was running low and Player A, whose turn it was and who had no chances of victory at all, could effectively decide the game by buying penultimate Province and a Duchy. That way, I wouldn't have enough VP cards to buy to catch up with Player B. However, Player A, whatever his motive was, bought only Duchies, which enabled me to Double Province and tie for first with Player B.
So, how should I and Player A have played in our respective cases? Was it right for us to play our best, or was it more important not to mess up with the players vying for victory. I personally think the former is more fair, but I've met people who don't share this opinion. Hence, I'm asking - and I'm interested in hearing your opinions on this subject.