I don't know... was Mordred historically the "bad guy"? In the first known mention of Mordred, in the Annales Cambriae (ca. 960-980) there is no indication of an evil Mordred, only that "Arthur and Mordred fell [at Camlann]". They could have been enemies, they also could have been fighting side by side. It was much later that Mordred became Arthur's enemy (like in Geoffrey of Monmouth's History of the Kings of Britain).
Mordred's turning to evil in the series is quite understandable, but was Arthur really to blame? Did he have another choice? Kare (Mordred's girlfriend) did after all try to murder Arthur, and she would try it again give the chance. So I think Arthur did what he had to do, even thought it may not have been an easy decision for him.
For the love of Camelot!