HE RAPED AND KILLED HIS OWN DAUGHTER!

HE RAPED AND KILLED HIS OWN DAUGHTER
I accidentally stumbled on a TV programme and a few minutes into the programme, I discovered that I was actually watching a true life crime documentary. A man in America had just killed his only biological daughter for the insurance payout of $10,000.00.
I cannot comprehend why  and how a father can kill and rape his 13 year old daughter, his flesh and blood for money. Why will a father think of killing and raping his own daughter? Can somebody give  reasons for this?