Huh! In retrospect, a lot of elements form Singer's films kind of hinted at the sort of subconscious ideation which drives people like John Wayne Gacy.
Not just the opening scene in Auschwitz, but also the opening scene of The Usual Suspects, where the cast are arrested, wrongly accused, and bond over the experience. Before being blackmailed by a liar/traitor in their midst, not to mention the flashback where Kaiser Soze recalls murdering his entire family when his crimes came home to haunt them.
He spoke of it as a demonstration of his "Will." However, it's also consistent with a Family Annihilator. Also, playing lame duck to put his victims off guard, and not see him as a threat was a favorite tactic of Ted Bundy. (As copied by Jame Gumb in The Silence of the Lambs.)