There is a way of telling the story of Camelot in which a fine thing is ruined by a few bad people: an adulterous knight, a scheming half-sister, a treacherous son. It is the version most retellings reach for, because it has villains in it and it lets the reader be on the right side.
It is also not what the texts say. Read them in order and a different account emerges, and it is considerably bleaker: every organ that makes the order great is the organ that ends it. Not a flaw in the design — the design.
1. Founded on a title that was never single
Malory's Arthur is made king twice, by two incompatible mechanisms, and the book never reconciles them.
There is the stone, which is election by grace: Whoso pulleth out this sword of this stone and anvil, is rightwise king born of all England. Nobody chooses him; a claim is written on an anvil and he happens to fulfil it while running an errand for his brother.
And there is Excalibur, which is a loan. An arm comes out of a lake holding a sword, and the Lady of the Lake gives it on the understanding that she may ask a gift in return, later, unspecified. Geoffrey's version of the same weapon is best of swords, that was forged within the Isle of Avalon — the otherworld arming the crown from the very first page of the tradition.
A throne that is simultaneously earned, inherited, granted and on loan has a fault line at its base. And the loan is called in: in the last book the lake takes its sword back.
2. Constituted by an oath it exists to break
The Round Table is engineering. Wace, who invented it in 1155, says why: so that no man could boast that he was exalted above his fellow. It solves the problem of precedence among violent men.
Malory then gives it a constitution. The knights are charged never to do outrageousity nor murder, and always to flee treason; also, by no means to be cruel, but to give mercy unto him that asketh mercy — sworn annually, by everybody, so that nobody in that hall can claim not to have known.
And the eighteen books that follow are the record of the breach. Gawain kills a lady three chapters after the oath is sworn. Balin has already sterilised three kingdoms. Lamorak is murdered four-to-one by men who keep their seats. Lancelot commits treason in the most literal sense the law recognised.
The institution does not fail to prevent the violence. It assembles it: gathers the strongest men in the country into one hall, binds them by an oath, and gives them nothing to do for most of the year.
3. Aimed at a good that outranks it
The order's supreme object is not the kingdom. In the Vulgate a knight can say of his own sword that it is the thing I prized most except for the Holy Vessel called the Grail — worldly power explicitly subordinated to a sacred one.
An institution whose highest aspiration lies outside and above itself has, by design, subordinated its own survival. So when the Grail finally comes to Camelot, the knights do the correct thing by their own lights and vow to go after it, and the king is not exalted. He is grieving, in the same chapter, four paragraphs later: ye have nigh slain me with the avow and promise that ye have made; for through you ye have bereft me the fairest fellowship and the truest of knighthood that ever were seen together in any realm of the world.
A hundred and fifty ride out. Fewer than half come back. Nobody did anything wrong.
4. Held together by a love that will crack it
Chrétien invented the affair as a compliment, and was precise about its mechanism: Reason, which dares thus speak to him, reaches only his lips, but not his heart; but love is enclosed within his heart. Love overrules reason, and the result is the greatest knight in the world.
The Vulgate kept the love and drew the conclusion Chrétien had avoided: the passion that makes Lancelot supreme is exactly what disqualifies him from the Grail. The best knight alive is built to fail at the one thing that matters most.
And Malory — who wrote the war, the burning, the exile and the ruin — will not condemn it. He stops the narrative in Book XVIII to defend it: Right so fareth love nowadays, soon hot soon cold: this is no stability. But the old love was not so.
That is not confusion. It is the most honest position anyone in the tradition takes: private fidelity is a real good, public order is a real good, and in this story there is no arrangement that saves both.
5. Wounded in a way that maps onto the land
Sovereignty here is a body, and the body's wound is the country's famine. Balin — not a villain, a decent man cornered without a weapon — takes a spear off a wall and strikes King Pellam with it, and Merlin tells him what it cost: for the dolorous stroke thou gavest unto King Pellam three countries are destroyed, and doubt not but the vengeance will fall on thee at the last.
And the same logic runs up to the king himself. Geoffrey ends with Arthur wounded deadly, borne to Avalon, and Britain begins its collapse in the same sentence. The kingdom has a single point of failure and it is a man.
6. Designed to defer its own ending
It does not even get a clean death. Geoffrey's Arthur is taken away for the healing of his wounds — gone, but not said to die. Malory reports the epitaph and refuses to vouch for it: many men say that there is written upon his tomb this verse: Hic jacet Arthurus, Rex quondam, Rexque futurus.
A promise of return is also a permanent deferral of the reckoning. The order never has to account for what it did, because it is not over.
7. Doomed by a foreknowledge that saves nobody
And here is the clinch, because it makes the whole pattern deliberate rather than unlucky.
Camelot is built by a man who knew.
Merlin foresees all of it. In Geoffrey his prophecies are an instrument of state — the Boar of Cornwall shall bring succour and shall trample their necks beneath his feet — and they work. In Malory the same gift is a catastrophe. He arranges Arthur's conception knowing what it costs. He tells Arthur that the child born on May Day will destroy the realm, and Arthur commits an atrocity trying to prevent it, and thereby causes it. He tells Arthur, out loud, before the wedding, that Guinevere will love Lancelot. Arthur marries her anyway.
And Merlin knows exactly how he himself ends, and announces it, and walks into it — following a woman who is ever passing weary of him, and fain would have been delivered of him, teaching her the craft she will use to shut him under a stone.
The institution was designed by a prophet who had seen it fall, and he built it anyway, and then demonstrated on his own body that knowing the ending averts nothing.
That is the thing to hold on to. Not that Camelot was betrayed, but that it was constructed — knowingly, by people who had been told — out of materials that could not do anything else.
The clinching image
If you take one scene from all of this, do not take the sword in the stone. Take Merlin walking into Nimue's trap with his eyes open.
The maker of Camelot, who foresaw everything, unable to save the one life whose foresight was total. The kingdom is his cathedral, and his own foreseen, unavertable end is the proof that foreknowledge is a tragic faculty and not a saving one.
Then let the oath and the Dolorous Stroke show the same logic at the scale of the whole order.
A note on this argument
This reading is available from the texts, and it is not the only one available from them. The Queste would say the order fell because it was not good enough, and mean it as a vindication rather than a tragedy. Wolfram, writing at the same moment, would deny the premise entirely: his wounded king is healed, his hero fails and is allowed to come back, and nothing has to be emptied for the sacred to be reached.
So take the above as what it is — one argument, made from a particular selection of voices, with every step of it linked to a passage you can go and check. That is the only kind of argument this material supports.