Every once in a while, I notice mine going at it, kind of like wrestling. I've never actually seen them kill another one. But, seeing as how they're opportunistic feeders, it's not completely surprising. I have seen one get a snail though.
I have one that took out a smaller nassarius. I stress smaller because he obviously didn't care for the shell, as the crab is still alive and the old snail shell is still empty and laying there right where he left it!
Is it a newer tank? Maybe there's not enough "stuff" for them to scavenge?