Most of the bettas I've had over the years weren't that good at population control. They only seem to get the newborns or babies that have something wrong. Plenty survive. The hunting urge also differs a bit from fish to fish. The one I have now ate 3 immediately after being put in the tank and I see him stalking constantly though the fry seem to have caught on that this isn't a friendly fellow.
I wish I could figure out another fish that would eat more young but not cause a total massacre. Maybe when this one passes I'll try 2 or 3 females instead of a male.