When I was commissioned to write a true-crime book about Thomas Gilbert Jr., who murdered his father, a hedge-fund owner, for cutting his allowance, I had no idea it would make me question the nebulous part mental health plays in the American legal system.

Blond, blue-eyed, and six-feet-three-inches tall, Tommy, as he was known to everybody, grew up in extreme privilege. He had an elite education at the ritzy Buckley and Deerfield schools before following his father and grandfather to Princeton, where he majored in economics.