When I wrote it, Catron, I did so simply because that was how I myself had always 'heard' it. Your question having raised the possibility that I'd got it wrong all these long years, caused me to delve into The Oxford English Dictionary. (I'm only too well aware that one can have lifelong mistaken ideas!)
There, I find the earliest use of the phrase 'hung and drawn' - minus the 'quartered' - dates back to 1320, while the full 'hung, drawn and quartered' did not appear until about a century and a half later. I'm claiming, therefore, to have been 2/3 right!
The whole of the dictionary's entry is fascinating re the distinction between hung and hanged. One key element is the dialectal one, which reveals that 'the murderer was hung' is generally specific to speakers in the south of England. This rather supports my contention that - whilst hanged is now considered the norm - hung is not actually incorrect in terms of usage.