In my theory, all meat eating dinosaurs were "somewhat" vicious. No matter how big or small they are every carnivore are considered fierce in their own way.

A recent study suggests that the tyrannosaurus might have been a scavenger, rather than a hunter, making them less violent than they are usually typecast as. From what I understand, though, the study is still in the early stages. Could go either way, but I thought it was interesting.

Some species like velociraptors sound like they were really violent though, and not just when they had to eat, but for sport too. So they were probably the "meanest" of the choices here.

All just theories though. We don't know for certain, but it's interesting to think about.