Basically, John List murdered his mother and his entire family over the course of a day. He was a weirdo and they lived in this huge Victorian mansion, and after killing them he dragged their bodies to the ballroom and laid them out.
No one knew why exactly he did it, although he was very religious and said he didn't commit suicide at the time because he wanted to be able to get to heaven and rejoin his family. He was in financial straights and the reason for their huge house was presumably his wife's money, from what I recall. He actually went so far as to pick his son up from a soccer game in order to take him home and shoot him
He'd already alerted the school that he was pulling the kids out for a family vacation, and canceled their mail, so it was quite a while before they found the bodies.
He was wanted for seventeen years before New Jersey police asked America's Most Wanted to air his story. He was spotted living with a new family in Arizona and arrested, finally sentenced for his crimes. He never really seemed to regret it.
This was a very famous case that brought a spotlight on family annihilators.