Most reef aquarists include some sort of hermit crabs to eat algae, scavenge for leftover food, etc.
Over the years I had many different kinds.
Currently I have some of these guys, but I have no idea what their exact species name is, perhaps someone will recognize it from the picture.
They were smaller when I got them, but they grew.
I think all hermit crabs kill snails, and eat them and take their shells, these are no execeptions, although they are probably not the worst as far as hermit crabs go.