No, you shouldn't. The game would end, and you would not improve your position by buying it. Your correct move is to not buy it. If you do buy it, it's a strategic blunder (possibly from a situation in which your chances of winning were effectively zero), but it's hardly kingmaking, at least in my view.
This is paradoxical. You don't improve your position so you don't buy it. But as I have supposed, you don't have the chance to win if you don't buy it either. So you can't improve your position by not buying it. Why is the correct play to prolong the game and just wait for the inevitable?
Okay, you absolutely, positively will not improve your position if you buy it. You know this for fact. If you do not buy it, your opponents could blunder horribly, and you might improve your position. It could be that you lose either way. Yeah, let's say there are not enough points left for you to improve your standing. Should you buy it? I mean, I would still say no, just in case you overlooked some Curse-giver, or Saboteur, or something. Anyway, I think that situation is rare. In most cases, buying it or not buying it will be better or worse for you, and you base it on that.
The point is that in games with any number of players there are situations where your play just do not affect yourself that much. In a game with more than three players, however, that play can affect the other players much more than it affects yourself. In this case the other players have no way to control their fate; who wins depends on your decision which is irrelevant to yourself.
Yes, and it's fine to make moves that disproportionately affect certain players. I mean, look, if you go after Possession you really hurt the guy who comes after you, and not the other guy. And if you buy Possession because your thinking, "I want to screw this guy over big!" then yeah, you have a problem, whatever. But you probably bought Possession because you thought it would be best for your position in the game. So the player after you suffers more than the other guy, but it's still strategic, not kingmaking.
Either strategy can get you to a win. And in this example it is indeed likely that whatever strategy you choose that strategy will win out. So your choice entirely depends on which other player you'd like to help.
Your choice depends on which strategy you think has a .000001% chance of beating the other strategy.
There is nothing wrong as long as you are playing to win yourself. The problem is there are situations how you play does not affect your own winning chance, but still affect others disproportionately.
I agree that if people are not playing to get their best possible finishing place, you will have problems. But if everyone is playing to win, most situations do affect your own chance of winning, perhaps slightly, but they affect it.