This sickens me. We had a 26 year old man in my hometown who was a year ahead of me in high school who was letting an 18 year old mother live with him. He claimed the 5 month old boy fell off the couch and hit his head on a table while she was at a class one night for college. The coroner said the amount of trauma the baby had suffered he surmised that the guy had thrown the kid against a wall. He died after being on life support for a day. The guy was arrested a few days later but released because the official report wasn't out yet to "confirm" he had killed the child, so they couldn't hold him. They actually had to have a cop near his house at all times because so many people threatened to kill him in return, but it also made sure he didn't run as well.
People like this are monsters, and there is very little to be done when they are that mentally damaged. They are the reason we have high end prisons and life sentences, and even that might be too nice.
The only silver lining in what happened in my town was that the little boy's organs were still able to be used for transplants, and healthy baby organs are extremely rare. They were able to save three lives that I know of from organ transplant from him.