Just something I heard from a friend (who works in crime scene investigation), but yeah, this is not a one-off. It's not common, either, but this type of abuse -- as mind-blowing as it is -- happens sometime. I think these human filth parents deserve the worst possible punishment.
All I can say is that mental illness is a clusterfuck. This world has psychopaths in it, and unfortunately, sometimes, psychopaths find themselves in charge of a baby. This is what happens when the adult(s) in charge have no conscience. Yes, there are people who absolutely do not give a single shit about the suffering of others.
Although it would be extremely hard to do, we should work towards creating a system that would prevent psychopaths from getting to a point of being in charge of anyone else. No, I don't know what the steps are, but I think that is an accurate identification of the problem.