Because they have nothing else to do and they're miserable so they want to try to destroy something that seems to cause happiness and joy. And they can't stand the fact that the idea of this story is so different. They're ignorant A*******.
they think twilight isn't all that great and they think they have the right to make fun of those who like it. what's worse if you have friends who hate twilight and they call all fans of twilight f***** etc, they are really calling you those names and you're thinking, 'you're suppose to be my friend and not care about what I like'. I wear my shirts, my sweaters etc and don't really care what they say. if they don't like it oh well.