The last episode about the boy who was a sociopath

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I was watching the last episode that said it was a new episode but I have the feeling it was a repeat. It was about a child murderer who supposedly killed this little boy and he was arrested, but he said the boy was too young for him. So the team went to question the family and noted that the father and mother hugged each other and cried as if they were afraid the other did it, but did not want to admit it. The father confessed but they did not believe him. So what happens is that they learn that the older boy killed his brother by shoving bits of toys down his throat and that he was observed going from calm into a maniacal rage. The team identified him as a sociopath, which was easy to see by the way he acted, being calm and not upset and acting even handed. So the question is since the parents did not say that he was in an accident and received a brain injury, nor that he was adopted from a violent family or that neither his father nor mother were drunks, child abusers, etc. how did he get that way? Or is this a case of poetic license like the dramas where a woman or man who was a University professor or in a profession that requires a lot of academic training winds in the Tv show as having Alzheimer?