Not a whole lot is really known about his crimes, unlike most killers he never spoke of his crimes and always claimed that they were hits for gangsters. The bodies left no evidence because of decomposition and they think there were many that turned to soil. They painted him as an average American douchebag, it is probably fairly accurate. I never thought there was a mystery really.
My question is why bother making a film about Gacy, he was never all that interesting or appealing in any way shape or form, to put it bluntly he is not worthy of a main character. Kudos to Mark Holton for giving it his all, but the script needed some work, and the director needed to bring some more of that great intensity that I know Holton has, this movie could've done with a little imagination.