The main point I made was that Gewirth’s argument turns on the Principle of Intervening Action.
This principle applies to all actions: it is not specific to rights. That means that, if we look at a case where there is a similar terrorist threat but no question of violating someone’s rights, we will get the same result: it’s the terrorists who will be responsible for the disaster. But that seems too strong.
Some people suggested restricting the Principle to cases in which rights are involved. The main question to ask about that strategy would be why the restriction makes sense. If the Principle gets the wrong answers about responsibility for consequences when the violation of rights is not at issue, why think that it gets the correct answer when rights are involved? Maybe there’s an answer; I’m just saying that’s the question.
Another apparently strange consequence of the principle is that it would hold Abrams completely responsible if he should decide that he has to torture his mother in order to save the city. But wouldn’t the terrorists bear some responsibility for that? As Sara pointed out, they’re the ones who put Abrams in the position of having to choose between his mother and St. Louis.
Finally, as Kiran noted, the principle is limited to cases in which another person brings about the consequences. If I have to choose between violating A’s rights and allowing hundreds of Bs to die of natural causes, say, as a result of a disease, the principle does not tell us whether I would be responsible for the hundreds of deaths.