Fact is, they're haters, they're overdosed with something called "haterade", it's like gatorade and powerade but only lowlifes drink them, and haterade boosts someone's anger and hate towards someone. These do not apply to everyone who dislikes or hates someone/something though. He's usually hated because he's not good, he's evil, he's emotional, he's a drama king, etc...Truth is, haters feed on someone's flaws and attack them, every little thing, they also exaggerate those flaws and make the person sound like a total shit head. Again though, these do not apply to everyone.
People don't understand what he's been through, they're unable to empathize with this because they know that what he's been through will never happen to them, so they don't bother. I'm pretty sure that if ANYONE on this site's family got killed by their sibling they would be worse. They just don't get that everyone's not Naruto, not everyone can bounce back from such a traumatic experience.
Despite what I said, I can also understand why they hate Sasuke, I mean, he's taking his revenge too far, that's not justifiable.
I made a thread like this once, but people are arrogant and they don't listen to people, I am arrogant myself, but some people take things too far.