I have a theory. Gene dies naturally from old age. He was 95 after all. His wife finds out and becomes very distraught. The pain from the loss of her soulmate was too much for her to bear, so she decides to commit suicide. She takes pills, lots of pills.
The dog gets hungry after a while, and since both of its owners are unable to feed it, it eats whatever it could find. The dog finds the pills spilled out of the bottle, and decides to eat it. Thus it dies as well.
Or the wife had a medical episode--that may explain the pills all over the bathroom--husband tries to hurry to her and falls--at 95 can't get up. And dog eats pills.
OR the 3rd Theory
Gene Hackman had a heated argument with the leader of The Illuminati, insults the black pope and the kingdom of darkness and goes back home
the Illuminati leader hires professional hitmen/acolytes to break into his house and murder Gene, his wife, and his dog... just like John Wick