Men/psychiatrists someone/anyone explain this to me

Why would this guy do what he did? He looked like a great dad? Enyewe it’s only God who knows what is in the hearts of men. I still don’t understand what happened to this family and I can only say Satan. 3 years later after watching every thing on this case I still don’t get it. This is why I can never have an affair with a married guy. How would I live with myself if he went out and did what this guy did?