The infanticide aming langurs was a news received with an incredible amount of shock for, althoug they are a species really close to humans phylogenetically, a great number of males kills juveniles.
Even though this happens in some other species, different kinds of explanations may be given, like regulation of population density, social stress, social crowding, sexual frustration, incest avoidance, or social bonding.
Recent studies promote a new hypothesis: Since they are not monogamic, males kill younglings from other males, so that the female may become receptive to him. 
This may be an example of how the "selfish gene" tries to protect its transmition to the new generation in the largest number possible.