We are not ducks, or polar bears or guppies or ants. No Arthur, we can find natural selection arguements in favour of all these things for humans.
Rape obviously improves the fitness of the male doing the rape.
Polar bear infanticide occurs when a male discovers a female with a cub. He kills the cub and hangs around waiting for her to ovulate again. Jay Hanson provided an arguement that these same incentives apply to humans, and backed it up with evidence about child abuse by step parents. Individuals are willing to cooperate in a war effort because the outcome is advantageous to their individual fitness. For the species, the best thing to have is genetic variety, as this gives the species more flexibility in the face of changes in the environment.