The kidnapper should precommit to kidnap a fixed number of people regardless of their propensity to keep to contracts made under duress. Like many precommitments, this harms the kidnapper if he actually has to follow through with it under unfavorable circumstances (he may know that nobody keeps such contracts, in which case he's precommitted to kidnapping people for no profit at all). However, it reduces the measure of worlds with such unfavorable characteristics, thus financially benefitting the kidnapper on average--if you know the kidnapper has made this precommitment, you can no longer use the reasoning you just use above and so you will obey contracts made under duress.
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?...
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.