Like everyone said, the likelihood of a hermit crab killing a snail for its shell is slim if you have extra shells in the tank. You have to realize that there is a lot of time and energy invested for a hermit crab to do something like that.
The largest hermit crab in my tank is about the size of a quarter, and I think my mexican turbo snails would probably start laughing if one of the hermit crabs came at one of them.