Martin didn’t start off as a bad person.
He was clumsy and shy, and bad at his job, but he was also caring and kind, in a very simple way. His acts of kindness were plain but sweet in their simplicity, and his demeanor seemed almost childish. I can see how that could make him endearing.
He was quite an uncomplicated character: all that we knew of him was that he’s clumsy, a bit squeamish, fond of insects and other small creatures, and that he wrote bad poetry. A perfect fit into the softie character archetype. You could find him cute, and develop an attachment to him - it was easy since he had no real flaws. But he didn’t have anything unique about him either.
That in itself is not necessarily a bad thing. Characters like him have potential to develop into someone interesting and complex. They have a lot of room to grow and change. But Martin’s biggest problem as a character is that he did not change.
The shift in his attitude in s4 was only temporary. He decided that isolating himself from everyone was the correct thing to do, so he did it. For a brief amount of time he became more quiet, started actively avoiding people, and began to work alone. But that was out of his own decision. Nothing in his point of view or values had to shift for him to take this course of action.
And that was a great moment to actually develop his character. That isolation, the decision to voluntatily sacrifice himself for others without even informing them of the situation - it could’ve made him stop and think. It could’ve made him take a moment to reflect on himself and his actions. It could’ve prompted him to realize that things weren’t so conveniently black-and-white as he had always believed them to be. That could’ve been the moment he altered his point of view. But it wasn’t.
You know what did happen? He got depressed. That’s it. He opened up to the audience about how much he’s scared of being forgotten, how much he cared about his mother and how she pushed him away, how out of place he felt everywhere he went. All of that makes the listener sympathize with him and feel bad for him, of course. But those are all issues he struggled with in the past as well. It’s nothing new.
And when he got out of the Lonely, when he more or less managed to pull himself together, he was exactly the same man as before. He might’ve seemed different to the listener, because only then did we learn about his self-esteem issues and his motivations. But his attitude did not change.
He remained the same, simple man we got to know him as. He still divided the world into “good” and “bad” categories, despite being shown on multiple occasions that the world doesn’t work like that. He’s in full-blown denial about being an avatar himself, because then he would have to either put himself next to the people he considers evil, or actually discard his childish point of view.
Because of that oversimplified way of reasoning, he ends up belittling and demeaning his boyfriend, even though he too went through something similar and ended up in a nearly identical position to Jon. Both of them have new needs connected to being an avatar, Jon’s are just more prominent. While Jon needed to extract fear and then speak it out loud, Martin felt the need to isolate himself. Because of that, he was also unknowingly isolating Jon, since he was the only person the man could talk to without being told nearly point-blank that he doesn’t deserve to live.
It’s like the writers were too afraid to properly develop him. After all, it was the sweet, plain Martin whom everyone fell in love with, and so even the slightest change to that beloved sweet cinnamon roll who can do no wrong would’ve caused riots! So they decided to half-heartedly entertain the previously discarded idea of Martin being Web-aligned. “Oh look, the guy whom everyone thought is an innocent pure-hearted sweetie is actually a great manipulator! Anabelle thinks he would be soooo good for her patron, she’s so smug about it!”
I think it was supposed to be him finally standing up for himself. “Look, your poor tormented sweetie is finally getting his revenge for all the mistreatment he had to endure! But he’s still fearful and insecure of course :) it’s your favourite guy beating up the people who were mean to him!” And he didn’t even do the beating himself, he just made Jon do it without much concern for the man’s feelings.
I understand that standing up for yourself is important, especially for shy and insecure people, but there’s a difference between defending your autonomy as a person and just being needlessly rude. I’m sorry, but I don’t think getting mad at your boyfriend for things he repeatedly told you were out of his control counts as “taking your own feelings into account for once”. Martin’s perspective doesn’t really extend past his own needs, and by season 5 it’s no longer cute. It’s just exhausting to listen to.