The Britons preferred Mordred to Arthur because with Mordred it was peace but with Arthur it was endless war.
~ Geoffrey Monmouth, The History of the Kings of Britain.
On the day I became King, I made a promise that I would stick to the Law.
~ King Arthur concerning his wife's execution.
You would kill your own SON?
~ Mordred to King Arthur in the final battle.

King Arthur is generally portrayed as the heroic king who saved Europe from darkness. However like many medieval kings he possessed darker attributes downplayed by later retellings. In Geoffery of Monmouth's History of the Kings of Britain, he's portrayed as a fierce warrior who clashed with the Emperor Lucius over tribute and whose wars disenchanted his people leading them to follow Medraut (Mordred).

In Sir Thomas Malory's Le Morte d'Arthur Arthur committed two chief sins. He unknowingly slept with his half-sister Morgause (Morgan le Fay in other accounts) because he did not know of his true parentage. Only after the deed was done did he discover she was his half-sister. She bore a illegitimate son named Mordred.

When Merlin gave him a prophecy that a May-born male baby would grow up to usurp him, Arthur rounded up all the babies born in May, and put them on a rickety ship and sent them all out to sea. There all of them drowned, save for one, Mordred who grew up to be his enemy and successor, as well as his murderer.

After the affair between Lancelot and Guinevere was revealed, Arthur sentenced his wife, Guinevere to burn for treason even when many of his Knights objected against such cruelty. Fortunately, Lancelot saved Guinevere, but Arthur went to war against war against his former friend. He was usurped by Mordred while fighting Lancelot in France, but he sought to regain his throne He was met his end at the Battle of Camlann. He struck Mordred through with his spear Ron and was mortally wounded by Mordred's final blow.