Walking over him evidently would have been a good idea, yes. But, first, the lack of any hint (either before or after killing him) that you should do this is inconsistent with all the other quests (that I have completed anyway, which is a long way from all of them). More importantly, IMHO, is that the subsequent dialogue is not just completely uninformative, it's misleading. The only option I got when speaking with Talion was for me to say that I had not killed him, even though I had. When a user is faced with that what are they going to do? Go back down on the off chance that walking around where you killed him will fix the problem in some way, or just give up? I suspect in most cases the latter, and if the user doesn't have a save game they can revert to before they became infected (which would not be smart, but is entirely possible) they might give up on the game, not just the quest. On the other hand, if, for example, Talion suggested that he had heard the Lich was guarding something, and if you could recover what he was guarding he might be able to cure you, that would be a great lesson about exploring before leaving, and would also leave the user with a way out.