My friends and I have often had a discussion on this topic. What we conclued was that war that would have resulted from Elrond killing Isildur. Not that I am saying he shouldn't of done it, but Elves and men would be at each others throats because Elrond killed the King of men and also wasn't Isilidur's son staying with Elrond (or was that later) so Rivendel would have been attacked. Elves and men would have killed eachother off so all that would be left would be drawves and hobbits. I am not sure that would be a good idea.
