I leave small creatures in there, and let them take care of it. But if there is more then the hermits can eat, after they eat some, take it out, let it dry in the sun (or i think you can boil it?) and you can keep the shell and put it back in the tank. I never worried about taking snails out. But if you have shrimp or craps that are senstive to nitrate (nitrites?-i always mix them up) then you might want to take the snail out so it doesn't mess with your tank.
Chances are the hermits didn't kill him, the snail fell over or died and then the hermits ate him...at least i've never had a hermite stright up kill a snail.