Not sure, but I remember I lurked here for a little while before I joined and I could not stand The Monsters. They had the biggest egos and were so rude and over the top, it was tragic, But then ARTPOP happened and it kind of knocked them down a peg, which honestly I'm happy happened.
Nowadays most of them seem fine and there are some really nice and friendly Little Monsters on here who I get along with great. As for why they hate Katy I don't know, I don't think all of them do. There are trolls in both fanbases who just bait each other into arguments. It is what it is I suppose.
xox