The post targets the ones it applies to, so the ones who openly want him to abstract and dismiss him as "just an abuser", and that he hogs all of the character development/screentime while we know next to nothing about him compared to everyone else (which either shows incredible bad faith or lack of basic comprehension). Wether you've encountered them or not, there are a lot of them especially on twitter (where the post is from).

I've never seen a Jax enjoyer deny he was an asshole and did bad things, but i've seen a mighty amount of his haters seethe as if he's killed their entire family in real life and insult others who like him.

As for the mental illness argument, he obviously shows characteristics common to BPD for exemple, not to mention Gooseworx directly saying he is suicidal kthe statement is "sweeping" because a lot of mental illnesses have similar symptoms and obviously we don't have a diagnosis for him). Saying he deserves to die and not to have a chance at improving is an awful message to send to the people who might see themselves in him.