At first watch, I thought the movie was just OK. But the more I thought about it, the more I hated it for several reasons. To keep it brief, I'll summarize them as succinctly as I can:
1) It didn't feel a Joker movie. It felt like this was originally going to be a remake of either The King of Comedy or Taxi Driver, but then later was going to be a homage hodgepodge of those two movies, but then later it was decided that slapping the Joker name onto the project would make it more successful. Arthur Fleck bares very little resemblance to the Joker other than the clown make up and gaudy suit. Arthur is just a mentally ill guy who kills people who he feels have wronged him or who were awful people. And though the Joker is crazy, his motivation has never been about just killing people who are bad people or who have done something bad to him.
2) The movie is supposed to make you feel sympathetic to Arthur, but I felt it failed to do so for me. Usually these kinds of movies portray the antagonists as being terrible people who either tried to kill the main character, or who killed (or did something terrible) to someone close to them. But all the people that Joker killed did not feel like they deserved it. The three Wall Street guys on the train? They were douchebags, but they weren't trying to kill anyone. They didn't have any weapons. It seemed like just a regular beatdown like the kids in the beginning. The mother? She was kind to Arthur as far as we the audience have seen, and we know that she herself had mental illness. The former co-worker? He tried to help Arthur by giving him a gun to protect himself, but then he tried to cover his own tracks when Arthur was dumb enough to bring the gun to a kids party. Sure, it sucked that he threw Arthur under the bus but at the same time it was understandable. The talk show host? Yeah, it was kind of mean that he was making fun of Arthur on his show, but he's a late night comedian, that's what he does. It was Arthur's own delusions that propped Murray up as this great father figure while watching his shows. He did not deserve to get shot in the head for that. Even the two police officers who were trying to arrest him get put in the hospital after being beaten up by the people on the train after shooting someone on the train, which only happened because they were trying to do their job by pursuing a wanted murderer (Arthur).
3) The movie expects you to have sympathy for people with mental illness, but that apparently doesn't extend to the mother. We learn that she herself suffered from delusions, and had an abusive boyfriend who abused Arthur. It frames it as if she let it happen, but she was a mentally ill woman with a violent boyfriend. Either he was likely abusing her too, or even if she hadn't she was probably afraid of him and wouldn't have been able to stop him. She was mentally ill, working class, and lost her job due to her delusions. But Arthur shows no sympathy for her and kills her because she lied to him and because he feels that he let her boyfriend beat him. So all this talk about having sympathy for mentally ill people and not treating them awfully comes of as hypocritical when Arthur, in a later scene, says he feels good about killing his mother (when his two former co-workers come to give their condolences about his mother's death).
4) There are no negative consequences to Joker's actions, and a lot of the bad stuff was his own fault. He kills the three Wall Street guys in the beginning, but he gets away with it for most of the movie, and his actions seem to ignite a revolution in the city with the common people supporting it. His mother has a stroke because the cops questioned her, which I thought was going to lead to grief for Arthur and him blaming himself, but soon after he learns the truth about his mother and seemingly hates her enough to kill her, so he likely would have killed her even if she didn't have the stroke. At the end of the movie, he's freed from police custody and is being celebrated for his actions by the rioters. But, maybe that was just a delusion, but even if it was as we next see that he's in a mental hospital. But even that isn't so bad, because he said earlier in the movie that he preferred being in the hospital when asked by his social worker how he felt about their sessions.
5) The "twist" ending wasn't a twist and was pointless. If the movie portrayed Joker as the chaotic supervillain mastermind from the comics, and then the ending showed that all that was just a delusion and that he was just a mentally ill simpleton named Arthur Fleck who dreamed of being like that, that would have been a twist. If it were the opposite, that the movie portrayed him as being a sympathetic simpleton but that was just a delusion and he really was just a chaotic, cold-hearted clown prince of crime, that would have been a twist too. But the delusion Arthur and the "real" Arthur at the end are the same person in the same situation: a mentally ill guy in a mental hospital having sessions where he talks to a social worker (both of which look similar). So there's no real twist.
6) I simply don't agree with the theme that all the people deserve what they got, and this idea that it's society's fault. I feel that that is just entitlement and wish fulfillment. It's a hard truth, but none of us are born deserving love and kindness from anyone outside of perhaps our family. The idea that people are entitled to have everyone just accept all their weird quirks is just not realistic. Society is about having to subdue certain parts of yourself to function and get along. "Be yourself" is fine with family and close friends, but we can't accept every single person we come across to just like and accept your true self. And going back to a previous point, most people in the movie didn't seem that bad to Arthur. His coworkers thought he was weird, but none of them seemed to treat him horribly. The boss also seems exasperated with him, but he didn't fire him because of his weirdness, he fired him because he brought a gun to children's party. Thomas Wayne is portrayed as being a jerk to Arthur, but it seemed rightfully so since Arthur was the son of a mentally ill former employee that believed him to be his father, came to his home, stuck his fingers in his son's mouth, strangled his butler, and then stalked him to the movie theater and confronted him in a bathroom. The mother was shown as being kind and understanding to Arthur, even afraid of him when he became angry after finding her letter to Thomas Wayne. The talk show host was himself a comedian, and just doing his job as a late night talk show host, satirizing and making fun of things in the media and in the city. He didn't seem to hold any personal ill will towards Arthur and was kind to him backstage.
7) The movie seems to have the idea that everyone was wrong to treat Arthur the way they did, but the movie ends up proving them right. He was weird and strange and not the kind of guy you'd want to get to know. He's the kind of guy that you'd meet and think "This guy ain't right" and you know what? You were right. He has potential killer written all over him.
There were probably other reasons, but those are the main ones that come to mind as of right now.