I don't actually hate School Days but, personally, I just didn't enjoy the series for a number of reasons. The plot itself is quite cliché and most of the characters are despicable in their actions. It's pretty infamous for the main male character, Makoto Itou, who cheated on a lot of women and didn't really care about them at all.
School Days is typically hated because of it's extremely unlikable characters. In the end, you either hate them or find them annoying. They try way to hard to be artistic, but it just winds up inappropriate. Now, this is just my opinion and no one's required to agree, but I don't recommend School Days. It's just not worth the rage.
I haven't watched it either, but from what I've seen, it's pretty bad. There's this guy, Makoto, who cheats on all of these girls and thinks that it's normal. He doesn't take responsibility, and his bad choices lead up to the ending.