I’m no psychologist but from what she’s doing she seems to have a need for power and control, and surrounds herself with infants who are pretty easy to control. I would not be surprised if the killings occured after the victim began to cry (like any baby does) and she was unable to get them to stop, undermining her fragile self worth and sense of control and leading to a period of aggression, during which she kills the infant. Once it’s done and the object of her aggression is gone her mind clears up and she realises what she’s done, and enters a state of remorse. I doubt she does it because she wants to, I’m sure she’s a very disturbed person, and her mental health issues are what prevented her from seeking help and from stopping. Of course some of the blame lies with the people who were aware and let it happen. It’s like letting a hungry wolf into a daycare and doing nothing to stop it.