[@Gowi]
I don't think it's just that he's fantastically corrupt.  With Arthur dead, and no heir (unless Mordred rears his head), the realm was bound to break apart, unmaking everything g that Arthur, Lancelot, and the rest of the Knights of the Round Table fought for.  So, the murder plot was conceived to kill off everyone who would have played a role in the eventual civil war, ensuring the realm's stability, at least in theory.

It is vile betrayal, but done to pursue a greater good.