Yeah come back to me when you've seen a fox kill something five times it's size "for fun." That's what really struck me, was the fact that pack hunting large prey is a dangerous activity for the hunters, and these raptors were just slaughtering these Parasaurolophuses one after the other without eating them. Predators don't risk injury to themselves "for fun."You've obviously never seen a fox in a chicken pen.
Foxes, cats, chimps (I think) all kill for fun, not just for food.
But I think the answer is that the raptors were bred with an aggressive gene and the hadrosaurs were in their territory. I'm betting that's the answer.