gladiusdei:
How do you decide who wins? I understand some players would be willing to lose to another to make a good story. But many other players wouldn't be happy with that.
If the ground rules for the game is that character can die at another player's decision, then everyone involved should be the kind of person who is happy about it.
Here's one such set of ground rules I found:
http://www.imaginechat.com/creed.shtml
gladiusdei:
How do you resolve situations where a player believes his character should be able to accomplish something, but others disagree?
I haven't done any free-form, really, but I could imagine one approach would be "If it has been stated, then it has happened." Others can "disagree" but they can't prevent it after the fact. If they wanted to be in a position to prevent things, they'd have to be able to point to something they had stated that would prevent it. If X wants to prevent Y from shooting B, X needs to say "I position myself to be able to stop Y from harming B." If X has superspeed or a magic spell, maybe that "position" is on the other side of the football field, or in another dimension, but the point is that they've established their "fictional positioning" and it can probably be generally agreed that if Y tries to shoot B, X will be able to intevene in some way.
What wouldn't work would be for Y to shoot B and then for X to say "But I was in a position to stop that" when X didn't specify that. Maybe X has superspeed and is standing right next to B, but without that positioning X is out of luck. (Probably, any character with superspeed would have had some overarching description relevant to the story that always lets them react, but without that, well, I guess they're just not quite super enough).
So, basically, it could be (roughly) about precedence.
Which doesn't necessary leave X entirely at Y's mercy. X has narrative power too, and can potentially add to Y's fiction. Unless Y has established that they're a crack shot with an intimate knowledge of anatomy, there's potentially room for X to state that that the shot, while injurious, hasn't killed B outright. It probably depends on the game whether or not and what X can add on to Y's fiction. But that would be my preferred flow of things: you can't block fiction, but you can add on to it.
An exception I would make would be if a PC were taking action against another PC. In that case, my preference would be to let the targeted PC decide the outcome of that action. If the outcome somehow redirects it back to the first PC (or some other PC), that PC gets to decide the outcome. In theory, one could extend this to aspects of the game beyond one's own character. If, in the above example, B is somehow crucial to X, X could get to decide everything about how Y's action played out - until Y would be suffering the consequences.
Obviously something like that could be gamed by a dedicated person, or could lead to oddness even between well-meaning people, but I think the overriding expectation is that players will sort things out among themselves.