I made the mistake of adding some large shells for them thinking it would make them a nice home and they wouldn't kill my snails.
I didn't realise that they would also grow into them and I produced 2 mighty hermit killers that were eating all sorts including other crabs and snails.
My LFS advised me to put them to work in the sump so they are currently in solitary keeping my sump clean. I hear them banging around and clinking on the glass!