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.