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, but this interpretation in fact came later. In the original legend as depicted by writers Mallory and Geoffrey Monmouth, Arthur was shown to be just as bad as two kings before him, Vortigern and Uther. Although Arthur's legend was expanded and altered to turn him into a hero, the original version, created by Geoffrey Monmouth, had King Arthur as being dark, cruel, manipulative, arrogant and greedy. Monmouth changed this over time, leading to the depictions modern audiences are more familiar with.

In personality, Arthur is depicted as arrogant, reckless, and even greedy and power-hungry, for he was quick to take Excalibur from the Lady in the Lake. Further, according to legend, when Merlin gave him a prophecy that a June-born male baby would grow up to usurp him, Arthur rounded up all the babies born in June, and put them on a rickety ship and sent them all out to sea. Then, all of them drowned, save for one, Mordred who grew up to be his enemy and successor, as well as his murderer.

King Arthur was depicted as warmongering, and when he waged a "quest" for the Holy Grail it was little more than theft, as he planned to take it from the Fisher King, its rightful owner, and use it for his own goals, not for his people.

He was also incestuous, although unintentionally. He laid with his half-sister Morgan le Fay, because he did not know who she truly was, when they were born they were seperated. He only heard from Merlin that she was his family. He disowned Morgan afterwards, thus disowning two of his own family - Mordred was his nephew.

He waged war with his own best friend Lancelot when his wife Guinevere was abducted. He later sentenced his own wife Guinevere to burn for treason even when many of his Knights objected against such cruelty. Fortunately, Lancelot saved Guinevere, but Arthur waged war against his once-best friend. He was usurped by Mordred, but he sought to regain his throne and was ultimately killed at Camlaan in battle.