The least convenient world is one where there's no traveler and the doctor debates whether to harvest organs from another villager. I figure that if it's okay to kill the traveler for organs, then it should be okay to kill a villager. Similarly, if it's against general principle to kill a villager for organs, then it shouldn't be okay to kill the traveler. Perhaps someone can come up with a clever argument why the life of a villager is worth intrinsically more than the life of the traveler, but let's keep things simple for now.

So, let us suppose that N sick people is the threshold wherein it is okay to kill a traveler, and hence a villager. If it's good to do once, it's good to do anytime this situation comes up. So we have ourselves a society where whenever the doctor needs is in dire need of organs for N patients, a villager is sacrificed. If we scale it up to the national level we should have ourselves a proper system wherein each month a certain number of people are chosen (perhaps by lottery) for sacrifice and their organs are harvested. I should imagine an epidemic of obesity and alcoholism as people seek to make their organs undesirable and so avoid being sacrificed.

I find that a fair number of morality puzzles of this sort exhibit interesting behavior under scaling.