Comment 1 by VrijVlinder :

Yes I have also found that aspect difficult to accept. Why kill progeny at all ? The most simple reason can be that the genetics of the new male are superior to the one displaced.

No. Animals have no notion of whose genes are "superior" to anothers. Its not even clear how you would measure that. Is a gene for enhanced intelligence "superior"? Probably, but what if it also requires the animal to eat twice as much to feed the enhanced brain or what if (to bring it down to human terms) it makes the male such a nerd that its much harder for him to find a female to procreate with? From the standpoint of evolution killing progeny that are not your kin in order to give your kin or potential kin a better chance of surviving and procreating makes perfect sense. There is nothing controversial or hard to understand, its all a matter of selfish genes.