I also don't think Squall hated everyone. Actually I don't think he hated anyone, he didn't even seem to hate Seifer. He seems like a jerk and hateful in his conversations with people, but you can see what he's thinking most of the time and can see it's not that he is trying to be mean, he just has bad social skills. Like with Quistis, he doesn't reply like that to be an ass, he is just scared to get involved. 
  
Rinoa was not stupid to fall for him, he is really cool.