Looks like a typical Red-Eye rock crab (Eriphia spp). They are mostly scavenagers and will also eat small inverts. There mean little guys I would kill him or get him out before he takes out all your hermits and snails. That is an exceptionally mean looking one too. Good pic!
The long, pointy claws would suggest that he likes to pry things apart to eat. Last one I had with claws like that ate xenia till he went in with the triggers!
I have the exact same crab but without the stripes on the legs. He's very shy. I've been feeding him to keep him from eating everything else. He's my "tester canary" until I'm ready to put in other stuff, since he hitchhiked in anyway. I've dropped food right next to my featherdusters on accident, and so far he hasn't hurt them, just takes the food. I've been feeding him bits of bottom feeder tabs and pieces of canned shrimp. Although I do think I remember seeing more hitchhiker snails in the tank in the past, so maybe he's eating them.