Pretty much everyone in those Arthurian Legends is a despicable asshole. Percival is the only decent one among them, so he was of course the only one pure enough to get the Grail. Merlin is especially duplicitous and annoying, and that's saying something given his company in those stories. He sort of makes up for it be being loyal to Arthur (eventually), but by then Arthur himself is intensely corrupt. Its strange that modern people look at those legends as examples of Knightly Virtue, courtly-love and chivalry, when 99% of the characters involved lived up to none of those standards. You can look at it as a humanist tale of humans failing to be perfect, but I don't think the authors looked at it that way. They were used to their rulers being giant dickheads, and wrote their mythical leaders accordingly.