There's a good side and bad side to every sign. Unfortunately, the self destructive Scorpio makes it easy to see only the bad in them.
Scorpios come off as mean and sarcastic and bitchy because, well, they are. On top of that, their Water sign nature makes their insults cut deep. They know what hurts and they don't have a problem saying it. They're secretive and sometimes dishonest, and that's easy for people to see. Take it from one. But, it's all a defense mechanism. It's a wall made to keep out the people who will only bring heartache. People who get past this wall see the good in Scorpios, and they are often very good, patient, and understanding people themselves.
Anyway, Scorpios do this because they're the typical tough on the outside, soft on the inside type. They don't like the idea of getting close to someone because it gives that someone the power to hurt them. On top of that, Scorpios take pride in being self sufficient and independent, and think that independence is threatened when other people are brought into the equation. They're extremely afraid of the possible rejection that comes with letting people see them for who they really are. (In all honesty, it's my greatest fear. I only have one person in my life who I can call a true friend, but that fear still lingers in the back of my mind.) Scorpios are extremely sensitive and easy to hurt, so they make it hard for the wrong people to get close to them.
So, that's why people hate Scorpios. Because self destructive Scorpios make themselves out to be the bad guys. All a Scorpio wants is someone who can see past the bad stuff. They want to be understood, but at the same time, they're terrified of that. When they find someone who truly cares though, as you know, they're extremely loyal and good to that someone. :)