I have a male Beta fish and the filter does not really get all of the algae off the tank so i have bought three different snails and he kills and eats them all is this normal?

It's normal. Betta males are extremely territoral and protective, and even snails may bother him in his space.

Every sort of fish loves to munch on carrion, so don't be worried about that.

You should try to get an algae scrubber to get the excess algae off your tank and try to move you tank to a location where it gets less sunlight, because that is what cause algae outbreaks.