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.

You may think you know of King Arthur as the heroic king who saved Europe from darkness, but if so think again.

The original legend, depicted by writers Mallory and Geoffrey Monmouth, had Arthur being just as bad as two kings before him, Vortigern and Uther. Arthur was not any better than them, and in fact he is shown as arrogant and reckless. Also greedy, for he was quick to take Excalibur from the Lady in the Lake.

King Arthur, according to legend, started bad when Merlin gave him a prophecy that a June-born male baby would grow up to usurp him. Fearing his downfall, 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. And rightly so.

King Arthur was warmongering, he waged a "quest" for the Holy Grail which was little more than theft, 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, but he had no sign of repenting.

He disowned Morgan afterwards, thus disowning two of his own family - Mordred was his nephew. He disowned Morgan, and Mordred, and then waged war with his own best friend Lancelot when his wife Guinevere was abducted.

He showed no signs of love, sticking to the Law, and 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.

Although Arthur's legend was expanded and turned him into a hero, sent from God, the original version, created by Geoffrey Monmouth, had King Arthur as being dark, cruel, manipulative, arrogant and greedy. So dark that he would murder innocent children. Monmouth changed this over time, but only through pressure.