I'm pretty sure that by definition it does prevent a negative outcome, since "mutually beneficial" generally means that each person is better off than before the trade, meaning there are positive payoffs on both sides - I'm not sure where exactly you get a negative from there.
In the example of "give me what I want or I'll shoot you", you could argue that "X prevents Y from being shot" and "Y gives X what he wants" is a benefit for both sides, but that ignores the first part, "X puts Y in a position where he is going to be shot" is not mutually beneficial, so the salient part of the example (the threat) is not actually an example of mutually beneficial exchange.
Again, my point was that even if every exchange, in and of itself, is mutually beneficial, the overall society can still be unpleasant. But it should also be noted that mutually beneficial exchanges do not prevent negative results even directly, but merely improve the outcomes for those who are agreeing to the transaction; two parties can agree to an exchange that leaves them both better off than they were before, but harms a third party. Which doesn't even bring up that these exchanges are based on perceived benefit...
In the example of "give me what I want or I'll shoot you", you could argue that "X prevents Y from being shot" and "Y gives X what he wants" is a benefit for both sides, but that ignores the first part, "X puts Y in a position where he is going to be shot" is not mutually beneficial, so the salient part of the example (the threat) is not actually an example of mutually beneficial exchange.