It's literally his job. He's a specialist and he wouldn't have had a following if people didn't come to him to seek his educated opinion. Roger Ebert loved film. He just didn't like to be treated like an idiot and thus was critical, only wanting modern film to be better.You say that but most current reviewers seemingly find great joy in tearing down a product on a personal level. It's not just fan reviews either I've been on separate sites where authors write their review in multi paragraph format and only focus on how they felt the movie didn't live to their expectations or how much they disliked x's personality in real life etc. etc. Also I don't know that film critics are the biggest fans of film tbh, they're a fan of film when it fits their preferred style but anything else not so much, you're not gonna be able to sit here and tell me Roger Ebert a man who seemingly disapproves of around 80% of all films is a bigger fan of film than I am just because he feels the incessant need to tell others his opinions.
Also you can't read into whether they want to hate the film, you can't tell that from a review. Jared Leto (I assume he mean him) has been obnoxious and critics have to factor in PR and advertising too. Don't blame them.
About the subjective/objective part you mentioned. You're correct- film is subjective. Everybody is allowed their opinions and critics are the ones putting themselves out there for abuse for giving theirs. Film when put through the ideas of award ceremonies, reviews or group discussion though? Suddenly it's not so subjective. There are expectations and a right/wrong board of marking.
I never understood the point of forcing opinions onto others. If you like a generally disregarded movie then what's the point in trying to convince everybody else they're wrong? More power to ya.