Your adult male betta may eat eggs if they are unfertilized or if they have some other abnormality.  Additionally, if your male is not getting enough energy from his normal food, he may eat eggs. First-time fathers or overly aggressive fish may also exhibit unwanted behavior like egg-eating. Stress also contributes to this. Stressed-out fish burn more energy and feel the need to take in more food. If you're not giving them more food, they'll find it in another (cannibalistic) way. Ensure they have enough foliage to provide them with hiding spots to destress and keep the tank away from the loud and bright parts of your home.