I liked him, he was just portrayed in a bad light so when he turned out to be good, some people just didn't accept it

He was an awful person, but a great character.

but nobody hates them

I hate them, and several other people too. Don't assume you know everyone and their opinion, because you don't

Fred and George never did anything too bad. They got into mischief, but they were never cruel. Snape bullied children, and he was a death eater.

Fred and George never did anything too bad. They got into mischief, but they were never cruel.

"Getting into mischief" is irresponsible, and breaking rules just for the sake of being able to is cruel because it endangers their peers. So yes, they were cruel.

Being irresponsible is different from being cruel. Snape intentionally caused harm. Fred and George never caused any serious harm to anyone.

Fred and George never caused any serious harm to anyone. 

They easily could have. Having a reason to back up cruelty (Snape) is better than doing it simply for entertainment (Frrd and George).

They would never intentionally harm anyone. Snape would.

Fred and George do. Regularly. They've tested their Skiving Snackboxes on children, aware that they could be hurt by the side effects or if it went wrong, just for their own ambitions. Not to mentioned all pranks end of hurting someone, and they do them willingly.

@Icecreamdif But, what about other Death Eaters? Or Bellatrix, they intentionally hurt and kill people. Even the good. Molly killed Bellatrix, Dumbledore killed his own lover. Ginny petrified people; intentions of killing. Does that mean anyone that hurts or kills people is evil?

What if they were driven from emotion? Or maybe from what side they were on? Perhaps possession? It just depends on why they did it, maybe a good reason, or maybe a bad one