Hermit crabs

Are there safe and dangerous hermit crabs and if so how do you tell them
apart? I picked one up at the beach a couple of days ago and seriously
thought about bringing it home for my tank but changed my mind and
thought I'd ask here first.

No, I don't know what sort it is but it's leg colour was boring, sort of
light brownish. It was using an enormous shell though, about 7cm long -
someone said it was a pippy (? if I've got that right) shell. They were
in abundance where I was and I thought it would be rather nice to have
something in my tank that I collected myself.