Keep in mind, he was 95 and was forced into retirement by health issues over 20 years ago. If your base rate is based on two healthy people in their prime, that's a little different. It could be as simple as he found her dead or dying, tried to get help and had a heart attack from the stress or simply just fell. Reports are they weren't found for two weeks, which sadly could explain the dog.

Tragic situation all around, but I'm not ready to use a probability based argument to insist on foul play.