Yes i think the motive is an interesting one, i do remember one quote of her saying it would be “cathartic” for her “to see a living baby in the space previously occupied by a dead baby”. That as a stand alone comment isn't hugely worrying, could even be religious, but with the evidence and context of this case, her behaviour is building a picture (with the parents, the note etc). That comment could be seen differently.
If she is guilty, Is it possible she got some sort of high from that, one baby going and a new one coming in. There is something known as the psychopaths high, which they get from killing, or other extreme activities.
But who knows.