This reminds me of Chris Watts when he was interviewed by the local news after his pregnant wife and two little girls went "missing" (they weren't missing, he knew exactly where they were, the wife was dead in a shallow grave and the little girls in oil batteries, where he had killed and placed them in the early morning before).

Chris thought he had gotten away with it, and he was happy that his "old" family was gone and he could start a new one with his mistress, so during the interview he was constantly smiling and laughing, making jokes, etc. Then he would "act correctly" when necessary: look sad, shake his head, plead for his family to be found, etc. It was all an act, just like this guy.