Why are there conspiracy theories? This is the account I read. His personal physician said that he had heart issues. He went out hunting on Friday, attended a party, went up to his room early. He was found in his PJs in bed, dead. The ME spoke to the police officers, and based on their confirmation that he was dead, and no doubt looking at him, she determined he was dead. There was an original cause of death as a heart attack, but was later changed to natuaral causes. He was 79 and his family did not request an autopsy. No doubt they were aware of his health issues. We are all going to die sooner or later.
In case anyone is thinking there is no precedent for the President making an appointment in an election year, this is the Precedent. Ronald Reagan appointed Justice Kennedy in an election year.