A few things that I (as someone who doesn't follow Twitter) have heard about from anecdotes and from the news about why people dislike him:

- He called some guy who worked for him a pedo for no reason, which he was sued over

- Took over Twitter and promoted right wing tweets to everybody, unbanned far right accounts and sued critics who said he did so

- Started promoting far right ideas, like when he retweeted "Interesting" to some tweet of a 4chan post saying that high-status, high-testosterone males are the only ones who can think freely and should be the only ones who can vote

- Took the side of the right-wing rioters who attacked mosques in the UK, saying civil war is inevitable

- Just seems to insult companies for little reason - advertisers who leave him, and Apple

There has been kind of a slow back and forth. Some people who liked him were miffed about the pedo thing, but they didn't hate him. But he has just kept doing things that some people hate.