Being kidnapped isn't that big a deal. Are you saying that he should just kill everyone who isn't known to keep contracts made under duress? If "he" is a large organized crime syndicate or a government or something, that might work, but there's no way one person could kill enough people to make it worth while to start paying people to kidnap you just because he might be the one getting payed. He'd have to cooperate on the prisoner's dilemma with all the other kidnappers, who are themselves defecting from the rest of society. Why would he do that?

There's a reason for the idea of fairness. Consider the ultimatum game. There's a Nash equilibrium for every strategy where one player will accept no less than x points and the other no less than 1-x points. It seems like you could demand 1-ɛ and they'd have to accept the ɛ because it's better than nothing, but by the same logic you wouldn't be able to ask for more than ɛ because they'd demand 1-ɛ. So you pick a schelling point and demand that much. You demand half. They demand half. You agree to split it evenly. If they demand more than the schelling point, you give them nothing. If there's some reason that the schelling point isn't completely obvious, you might give them some benefit of the doubt and probabilistically accept so you don't both get nothing, but you make it unlikely enough that them demanding more than the schelling point is not a viable strategy. This is what fairness is. It's why shouldn't agree to unfair deals, even if the alternative is no deal.