Reason 1 - She Killed Shannon

I was beginning to like Shannon. She was taking proper care of the dog, worrying about Walt and generally contributing to the group. But I have to admit that is not the real reason. Killing Shannon upset Sayid. No one messes with my Sayid. I really wanted him to rip Ana Lucia a new one - at least give her a black eye or something. I was very disappointed when he didn't.