I think it was a murder committed by the now-dead wife resulting in a murder-suicide. I think the wife literally bumped him off for his money by just maybe simply knocking him down and leaving him there to die. She had 80 million reasons to do it.
https://www.celebritynetworth.com/richest-celebrities/actors/gene-hackman-net-worth-2/
He may have decided to leave his money to his kids instead of her which would motivate her to do it instead of just waiting for him to die.
She then was overwhelmed by grief and the gravity of what she did and then committed suicide.
This is all pure conjecture and speculation of course.