I honestly can't wrap my head around the hate for this game
Really? I think it's fair to say that it's overblown, but killing off the main character in the series is a purposefully controversial ploy to try to emotionally invest the audience in your narrative. People are going to be understandably upset if the original game meant something to them. And I think it's a cheap foundation for what could ultimately only amount to a melodrama, because it relies on the quality of the original narrative to evoke a response instead of earning one through good writing. You could argue that Joel's story wasn't over and that his actions demanded consequences, but that's a cop out. That story didn't need to continue, and it's more powerful to leave it as is with humanity's fate inconsequential to a man who got away with murder to protect someone he loved. And it's ultimately to serve some trite revenge plot? I think you can appreciate how that might be insulting.
There's also an element of certain people being upset because they replaced the prototypical male lead with modern gender and sexual identity politics. Yes, they're bigots, but in a world where a not insignificant amount of men get upset whenever women in video games are portrayed as more than busty accessories, Naughty Dog replacing the invested masculine player surrogate with a sociopolitical message is purposefully egregious.