Have you ever heard of a baby rat killing a hamster?

My dwarf hamsters and baby rats used to play together just fine until the other day when one of the hamsters attacked one of the baby rats. The rat became angry and bit the hamster on the back of his neck. It all happened so fast that we didn't have time to stop the rat. When we finally separated them the hamster was twitching on the floor in a puddle of blood . It was so morbid! Is it common for rats to be violent towards other small animals? Or is my husband's rat just plain evil?