everyone here seems very enthusiastic about mordred, why
For me it’s the tragedy of his story! How he was born already cursed (in Morte d’Arthur, Arthur has a dream about his son killing him and so he decides to take all the babies born the 1st of May and kill them, but Mordred survives) and how differently you can adapt his story (in Morte d’Arthur he is a villain, but his actions can still be read as sincere, in some versions he just wanted to stay with Guinevere and love her, in others he was maybe one of Arthur’s companions etc.).
I love difficult family relationships, and so I love Mordred!
Putting in my two cents, I have a few reasons:
1. As a character, he has the potential for a lot of complexity. He’s in the center of the action, and has ties to most of the other major characters in various ways, which gives him a unique vantage point on a lot of the popular stories. I enjoy complex and fraught relationships, especially where family is concerned, and if Mordred has one thing in spades, it’s family drama. And like @galahadmordred said, he’s a very tragic figure, which is always fun to read and write about.
2. His motivations and actions – and even his pure existence – have become more sympathetic over time, as the climate of our society has changed. Referring specifically to everything after the Vulgate Cycle, wherein he’s (usually) Arthur and Morgause’s illegitimate son, his very existence would likely have been enough to make him unsympathetic to a contemporary audience. He’s sometimes written with more egalitarian leanings (inasmuch as a character in that time period and setting can be considered egalitarian), and speaks out against Arthur’s tendency to weight his laws unfairly in the favor of people he likes. (I’m particularly thinking of the Lancelot/Guinevere debacle, which, in some versions, is what lights the spark of Mordred’s defection.) Originally, that would not have gone over well with the target audience: betraying one’s sovereign was obviously treason, and made one the most vile of villains. Nowadays, we view such behavior in a more positive light, because we understand that the government should serve the people and not the other way around. At least, ideally.
3. He can often be read as queer-coded, and my queer ass eats that shit up, even though it can be Problematic™ because I’m starved for representation.
Arthurian Legends 