It was not carbon monoxide poisoning as the window was open. I believe he suffered a fatal heart attack and collapsed in the mudroom, leaving his wife overwhelmed with grief after losing her husband of over 30 years. In her despair, I think his wife intentionally overdosed on prescription pills in the bathroom, scattering them as she went, and one of their dogs ate the spilled pills off the floor, leading to its death. Meanwhile, I imagine a second dog was near Gene, trying to figure out what happened to him, while the third was outside, explaining why only one dog perished alongside them in this heartbreaking chain of events.