There is no shortage of critics willing to pick up the metaphorical ax against poor writing, especially online. But I would like to do it from the standpoint of the morality tale and its value to society. Moral clarity is almost impossible to write well if you lack it yourself. Director David Lowry admitted, in an interview I can no longer track down, that he wasn't sure he could do the story justice. In style and atmosphere he has, but in substance he has failed utterly. Arthurian stories are morality tales as much as fantasy and the morals of A24's film are decidedly muddied if not opaque. I will focus on three aspects of the story which deviate from the original poem and make it utterly devoid of moral meaning. Spoilers ahead for those who want to watch the film.
Foremost is the character of Gawain. The movie begins with him waking up on Christmas morning in a brothel. This, admittedly, I knew going into the theater due to a couple of reviews I'd looked at beforehand. This wasn't a deal-breaker for me, I could easily imagine the journey of Gawain granting him the moral clarity of a man walking knowingly to his death for the sake of keeping his word. This did not happen. This Gawain eventually yields to the temptations of a lady in whose home he is a guest and whose husband had preserved his life and offered hospitality. In the original poem, as translated by the beloved J.R.R. Tolkein, Gawain resists the lady's advances. Here he finds only shame after failing to resist her. This Gawain begins a deeply flawed man and ends the story retaining several of these flaws. There was no need to change these elements of the story at all. the original Gawain was still flawed but does not fall to lust. he falls to cowardice and is marked by it so he will keep the memory of his failure as a warning forever after. No man is always and everywhere perfect in his conduct.
When the Green Knight first appears he hands King Arthur a letter bearing a challenge: Let whichever of your knights is boldest of blood and wildest of heart try, in honor, to land a blow against me. Gawain answers with the boldness and lack of forethought a young man typically has. Especially if the young man is still seeking to prove himself. We never again see this Gawain, save, perhaps, at the very end. He spends nearly the whole movie displaying cowardice in many flavors. Abuse of alcohol, fear of commitment to the woman he supposedly loves, turning his back when a thief pulls a knife on him, and inevitably shrinking away from the Green Knight when the time comes to be held to his word. At the very end of the film we see Gawain cast aside his fear and show himself willing to keep his word unto the point of death. More on this moment later.
Second, the presence of Gawain's mother in Camelot is confounding and her use of witchcraft even more-so. Christianity in this movie is window dressing only, kept for style but having no bearing on the plot or any of the characters. Morgan le Fey is Arthur's sister and nemesis in most stories. Arthur knows of her presence in Camelot and tolerates it, even embraces it. I may just be under-studied in Arthurian stories, but in all I've read she was never so chummy with her sibling. She it hateful and jealous of Arthur. Perhaps it's more of the "wokeness" of the creative industry infecting the film? How could a woman possibly be unjust? Or how could we make her unsympathetic? Worse, it is heavily implied that Morgan created the Green Knight and controls it. This makes the final moment of the film utterly confusing. Again, I'll hold off on describing the final moment.
Third, the lord of the castle near the Green Chapel. In the original poem, this man was the Green Knight. He had been disguised and protected by Morgan le Fey who had laid out this scheme to trouble Arthur and his queen on Christmas. In the movie version he appears to be some kind of hedonist deviant who was very interested in letting his wife prey upon Gawain, a man in his care. In the poem he was testing Gawain's virtue, in the movie he seems to have some kind of homoerotic designs on Gawain. Designs he meant to accomplish through manipulation and the twisting of Gawain's promise of mutual gift giving.
Lastly, the end of the film. When Gawain comes face to face with his executioner, he gives up the ax and kneels down to receive the strike to his neck. when the Green Knight begins his first swing Gawain flinches away. The second time, he asks the Knight to stop. The third time he asks if that's it, if there's nothing more to the game the Knight had challenged him to. The fourth time he runs away entirely. We then see Gawain's future after he runs, living the life of a coward and refusing to remove the magical belt he'd gained from the lady of the castle which would protect him from all harm. At the end of this sequence Gawain realizes that his escape was in name only. That he'd died the moment he ran. This life is then revealed to be an imagining of Gawain as he waits to be beheaded. He asks the Knight to wait. Gawain discards the magical belt that he believed would protect him and says he is ready. The Knight bends down to touch his face, in the same way almost every character in this story does for some odd reason, then gives the final line of the story: :Now, off with your head". This obliterates the original moral message of Sir Gawain and the Green Knight. the original story was a story about honor, courtly love, and the price of cowardice and dishonesty.
The moral of this film, as far as I can tell, is that no one should try to be great. If you behave honorably you will die and your story will end. If you do what's in your selfish interest, you will live a life hardly worthy of the name, then die anyway. It is an Arthurian tale not worthy of the name or genre it was made in. As a morality tale, it is uselessly opaque in its message or presents a message of utter nihilism. The visuals and music are of the highest quality, but the story is so divorced from its roots that it is utterly worthless for its intended purpose. What's worse, this will be the only version of the story most people will even know. A true pity.