Alien 3 supporter signing in. 
I'll be the first to admit that it is nowhere near a perfect movie, but it absolutely doesn't deserve the hate it gets. I can only come up with one reason why people tend to have such vitriol for Alien 3; As the OP points out, it simply wasn't the movie that people wanted. They wanted Aliens to infest Earth. They wanted the Marines, lead by Hicks, to kick their bug asses off our planet. They wanted heroics. They wanted a happy ending for Ripley and Newt. But they didn't get what they wanted, so they chose to hate it.

I'll be the first to admit that it is nowhere near a perfect movie, but it absolutely doesn't deserve the hate it gets. I can only come up with one reason why people tend to have such vitriol for Alien 3; As the OP points out, it simply wasn't the movie that people wanted. They wanted Aliens to infest Earth. They wanted the Marines, lead by Hicks, to kick their bug asses off our planet. They wanted heroics. They wanted a happy ending for Ripley and Newt. But they didn't get what they wanted, so they chose to hate it.