Every one in the society would wish to be supreme over others or to be in control of others. It is not different to animals too, when it comes to sexuality, males would always want to dominate. This is why most of them when they find out that there are infants born and they are not responsible they kill them.
This will enable the females to stop breastfeeding and start the whole cycle of sex life. By doing so, they also increase their chances of giving birth to young ones of their own genes unlike the previous killed that he might have taken care of but do not carry his genes.