Think of it this way. Say that a male individual has a mutation which promotes infanticide of other males' offspring. Because that female mother becomes fertile more quickly, the killer has a better chance of impregnating her and carrying his genes forward than if she were still nursing. Evolution works by increasing the frequency of particular mutations in the general population, and it does this by increasing the progeny of those individuals carrying the mutation - either by improving survival or, as in this case, by improving his ability to generate babies.

Remember, evolution doesn't care about improving the species, or ensuring its survival. In fact, evolution doesn't have any intelligence or emotions - although it seems as if it does, sometimes. It's all in the mechanism that Darwin described in his theory.