What makes you think that the rational hero wouldn't push the fat man?

From chapter 39:

Well obviously I'm not going to popularize a method of immortality that requires killing people! That would defeat the entire point!

and recall his anguished inner debate about whether he would, in extremis, allow himself to kill people on the other side in his war against Voldemort. But, for sure, that's not enough evidence to be certain he wouldn't, which is why I added:

(though, I dunno, he might).