This rhyme is based on an actual series of murders that took place in Edinburgh in the early 19th century, and was the inspiration for the Robert Louis Stevenson story "The Body Snatcher" (see the 1945 film with Karloff & Lugosi in supporting roles), and coined the term "Burking" (murdering someone and then selling their body "to science." Though you can probably add "involuntary organ-donation" to this as well.)

It's still a highly controversial issue to this day.