Kingsley poisoned the cold medicine. Stan stole it from his room.
A mistake. Kingsley had intended it for his wife.
Why was the watch an hour fast? Because it was still set on Chicago time and therefore belonged to Kingsley Mellon, as did the diamond ring, as did much of the cash, as did the fatal bottle of cold medicine. But let’s start at the beginning.
Kingsley was determined to kill his wife. She was older, owned the estate and controlled much of their wealth. He wanted out.
Kingsley bought the cold medicine and, while in his bedroom, poisoned it. He was hoping to lay the blame on an act of consumer terrorism. But while Kingsley was doing laps, the fired groundskeeper sneaked in, stole his watch, money, ring and, since he had a cold, the re-packaged medicine. This explains why Dinsmore didn’t seek help. To do so would be to turn himself in as a thief. Better to tough out the cramps and hope for the best.
When Kingsley discovered Dinsmore in his final spasms, he realized that the wrong person had gotten hold of the poison. Had he done nothing, he might have gotten away with the crime. But Kingsley panicked. He grabbed the ignition key, opened the trunk, retrieved the leftover medicine from a suitcase and disposed of it. Then he returned to the house and called the police.
Later in the investigation, the police showed Kingsley the suspiciously expensive watch and ring. At first he denied ownership. But Ellen Mellon identified both items and Kingsley was suddenly under suspicion. He confessed the very next day.