Apologies. You are quite right, I was indeed being "really blind" and pretty obtuse as well (though not deliberately so). I've now spotted that the original poster explicitly said to ignore alll chances that the fat man would fight back, and presumably that extends to other external costs, such as retaliation by his relatives, the law etc. My bad.

I've also commented on this further down this thread. I now find my moral intuitions behaving very strangely in this scenario. I strongly suspect that my original intuitions were very closely related to all these knock-on factors which I've now been asked to ignore.