User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Arthur to die? or not to die? at the VERY end?/@comment-4640114-20120902205138/@comment-4190137-20120903222325

Because it is Arthur's destiny to die at Mordred's hands. It is an inevitable event. So if it doesn't happen, that would be the most silliest and unrealistic thing the show has ever done (with an exception of them forgetting about Alvarr!).

The reason he has to die the first time is that we know the Battle of Camlann is going to occur in Series 5 (flashes of red sky in the trailer), and Arthur is likely to meet his doom there. But if Morgana takes his place then yeah...

In every legend, Arthur is eventually killed by Mordred, if that doesn't happen, then I wouldn't call 'Merlin' a variation of the 'Arthurian Legend'.