It happens in humans too. Although it is more premeditated in humans. Women have killed their own children to gain another male's affection. I am sure in the case with humans, genetics has no part because these decisions were made intellectually and are not innate functions of a human. Males have also killed stepchildren.

Every time a male gets killed, the genes of that lineage begin to disappear from the gene pool. I think that was the idea when kings etc ordered the killing of all the male babies. From the outside this would look as a way to prevent a group of people from producing warriors in the future. But the greater repercussion is the loss of genetics from that group of people. They likely did not think in terms of genetics but the result is the same.

Decreased variability in the gene pool.