I don't hate him, I don't know him. I have never watched an episode of American Idol. As to why some people dislike him so much, watch the video. See how good old Simon rolls his eyes when the contestant sends condolences to the victims of the VATeach shooting? That type disdain and smugness could be one of the reasons.
I'll be honest enough to admit to watching American Idol...after a hard day of work dumb tv is a guilty pleasure.
People boo Simon Cowell because they are told to, plain and simple. They don't hate him, they love him. And more importantly, hes the only judge who gives any feedback to the singers that is at all valuable. If you watch, you will notice that the singers barely listen to the other two judges comments - but they listen intently to what Simon says, because his advice is actually useful and honest.
As far as the Virginia Tech Eye Roll thing...if you watch the whole clip you will notice that Simon Cowell was not listening to the singer who was making making the comment at all. He was rolling his eyes at Paula Abdul, not about the V-Tech comment.