Mean ___ Emerald Crabs

I was just looking in my tank and noticed that my biggest mithrax crab was tearing my smallest mithrax crab apart. He managed to rip all of the little guys legs except for one and a half and the claws. Is this normal behavior. I have probably 4-5 in there right now in my 90 gallon. This is not the first time these two were all caught up. But this time the big one had the right hold and just ripped the legs right off. Now the little one is just scampering around the bottom. Not getting around as well...obviously. What should I do?? Is this too many? I didn't think so.