The whole review is worth reading, but here's a quick breakdown. He starts off by immediately spoiling the twist ending -- again, specifically to discourage people from going to see it. He then goes into more details about its flaws, including the acting, direction, basic premise, mere existence, etc. All standard stuff. But then in the last paragraph, he goes way over the line:

The Chicago Tribune
"IT'S ABOUT ETHICS IN FILM JOURNALISM!"

Betsy Palmer is the actress who plays the villain in the film, and yes, that is Gene Siskel disclosing where she lives in a major newspaper and encouraging people to write her hate mail. Palmer never got a single letter, but that's more a matter of luck than anything else.

Now, obviously this was in an era before the concept of doxxing and its dangers were really understood, and although stalkers were certainly around, it still wasn't a problem people took that seriously. (It would be a few years before anti-stalking laws started showing up.) But hate mail was still a known thing, and Siskel surely knew he wasn't inviting fun times upon his hated enemies -- who, again, were just people who made a movie.

It's a surprising ending. A reasonable initial reaction is that Mookie doesn't do "the right thing" hinted at by the movie's title. But there are a lot of "wrong things" in this movie, racist acts big and small by characters of varying degree of sympathy -- a black youth being choked to death by the police being one of them. The movie is, among other things, a portrayal of why reasonable people become angry enough to riot. But for at least a few critics, that didn't matter.

David Denby, writing in New York Magazine, had some reasonable criticisms of the story and its message, which he found somewhat muddled. But he went way past reasonable when he wrote: "The end of the movie is a shambles, and if some audiences go wild, he's partly responsible." In the same issue, Joe Klein ratcheted up the rhetoric a notch more, criticizing the way he thought police and white people were portrayed as the enemy, and warning "if black kids act on what they see, Lee may have destroyed his career in that moment." For some reason, he also tried to tie this into the city's upcoming mayoral election, wondering what effect this probably riot-causing movie would have on the chances of David Dinkins, a black candidate.

Lee, of course, thought this was complete garbage, and reasonably pointed out that black people can tell the difference between fiction and reality, and would also notice the very real anti-violence messages present in the movie as well. He also pointed out the almost complete lack of interest many critics seemed to have in the death of a young black man right before the riot; the fucking property damage was evidently more important to them. 25 years later, he was still angry about these reviews, and we can hardly blame him.

His problem may have simply been that he was ahead of his time. In the 1950s, America was still in a postwar mindset, and for the most part was looking for upbeat movies -- even Hollywood's monster movies of the time had flippin' Abbott and Costello in them. With Night Of The Hunter, however, Laughton gave us a psychological thriller about a sinister preacher menacing a widow and her children. Even the critics who judged the movie on its own terms weren't overly fond of it, and others were downright shitty. Here's The Chicago Tribune:

The Chicago Tribune
Yes, we always hate to have brutality and ugliness in our horror movies.

And The Pittsburgh Press evidently wanted its horror movies told without any of that fancy mumbo-jumbo like symbolism or creative camera angles:

The Pittsburgh Press
A jimdandy yarn of a review.

Whether it was because of the critics or the lack of people getting hit in the face with cream pies, the film was more or less rejected by moviegoers. Laughton was never given another chance to direct again, and it would be decades before we got another clear look at the delightful darkness that lies in all humans.

But about a week before the movie's theatrical run began, a bunch of anonymous IMDb users began giving it low ratings, which caused its overall score to plummet. This wasn't the only case; it's also happened to other indie movies, like Other People and Demon. There's no obvious reason it's happening, either. It's evidently one of those things which make the internet such a hellscape sometime.

This is a big deal for independent films. While a lot of blockbusters are essentially review-proof, in that they have enough capes and cowls and marketing that their opening day audiences are more or less built in, small movies don't have that kind of anticipation or marketing budget. They need good reviews, and IMDb is the biggest source of them after Rotten Tomatoes. Whether it's for the lulz or because they're trying to boost the comparative rating of their own grief-based indie films, some trolls are causing real damage here.