Primate groups are rife with infanticide. Now there's more evidence females kill each other's babies too.
Typically, it is the newcomer males who kill infants in a group. Infanticide usually happens in species where males leave their natal groups (the ones they were born in). At maturity they leave to form their own group or join an existing one. By killing babies, the new males bring the nursing females back into estrus sooner so they are able to impregnate them and sire their own offspring right away rather than waiting the 3-4 years it would take for the females to wean their babies sired by the previous male and become fertile again.
When infanticide was first observed in primates, scientists thought it was pathological. Later the careful detective work of primatologist Sarah Hrdy demonstrated that infanticide was an adaptive reproductive strategy for males.
I think the same thing might be happening with the discovery of female infanticide in primates.
When Jane Goodall first observed a mother-daughter pair of chimps who killed, in seemingly systematic fashion, the offspring of other females in their troop, she thought the pair might be mentally off - perhaps psychotic. Serial killers even! At one point, Goodall co-authored an article with an American personality psychologist in which they created personality profiles of all of the females in the troop. They showed that the two females (Passion and Pom) were different from the other females.
Even with this new observation of female infanticide, the behavior is unusual and not normal so far as anyone knows right now.
It all leads me to question whether this is abnormal behavior/mental illness in chimps or some sort of adaptive reproductive strategy in which the females are taking out the competition.
I think it is also possible that it could be both abnormal *and* adaptive, but doesn't warrant a label of mental illness. For aberrant behavior to be a "mental illness" it has to be maladaptive. There isn't a whole lot of evidence that wild primates have any mental illness. That's associated with captivity - a fact that would be totally unsurprising to Freud who believed that Civilization is what causes our discontent.