A really important distinction here is that most people with rat posioning use it to kill rats. Most people with heroin use it to get high.
I don't what all the posts say, but the autopsy said this from Nick Tosches The Devil and Sonny Liston (the only source I can find which quoted from the autopsy report) Las Vegas Sun--"Although traces of heroin were found in the corpse of former world boxing champ Sonny Liston, local experts have ruled the 38-year-old heavyweight died from natural causes." The autopsy itself includes this finding "an increase in fibrous tissue about the small blood vessels in the septums of the heart. The small arteries and the arterioles in the heart show considerable intimal thickening." So the physicians concluded Liston died from small vessel heart disease, as far as I can interpret this.