There have been a lot of new studies on why some mothers feel the desire to commit infanticide.
I hate to be too scientific and not just stick to "evil in the world," of which there is a lot to go around, but there are some chemical "switches" that some researches claim are flipped by altered thyroid activities in mothers. Read the new news story about mothers' breast milk carefully. You'll see the role of iodine, etc. http://www.newenglandpsychologist.com/leading/0109_coverpostpartum.htmlhttp://www.emedicine.com/med/topic3408.htm
It's horrible, but when we ask "why" I think we shouldn't rule out the possibility that the lady is sick, i.e. she's nuts. Well, at least then. Fortunately doctors have been working on treatments, dietary as well as drug, and we don't now how many of these situations have been avoided because of our current knowledge of postpartum depression.
Now I stand back to be assaulted for stating my belief that mental illness is an actual illness that we should treat.