I wouldnt think that your betta would do any harm to the apple snails. The fish would be stupid to take on something that is alot bigger then himself. As people have said, the most it will do it flare up and sometime nip he sell. Once the betta knows he is no threat then he should leavehim alone. As another person said, add everything you want to the tank before putting the beatin (Add betta last) this is make him think everything in the tank is not athreat but pat of the tank.
Jason... How long have you had the snails, could be just they died... I believe that a betta couldnt do that, and if he did then he is one smart fish, I suppose its free food for him lol... Why not replace them (As cheap as they are) and closly watch to see if it is the betta. if not asked the shop for some bigger empty shells for them. They might just have moved to find a bigger shell and is hinding because they cant find one... (Sorry really dont know how snails work)