@the3rdpoliceman, the narrator of the song is not john wayne gacy, it's sufjan himself drawing comparison to him. Though he isn't a serial killer, he is still a sinner and has terrible faults (in his mind), and thus is "really just like him". It is indeed a very powerful artistic statement to compare yourself to such a vile and horrible person. One of sufjan's best for sure.
The line 'And even more, they were boys, with cars and summer jobs/Oh my godddddd' is a lot more powerful when you realise the song's narrator is a serial killer. It's like he's aware of how awful he is but he just can't stop.
I wish there was more songs like this about serial killers. Most of them are death or thrash metal and they manage to sound violent, but not so chilling. Climbing up the walls by radiohead is an exception, but I don't think it's as good as this.
This song is featured on my "[URL=http://open.spotify.com/user/mic180/playlist/6K53a77MEIOWrXWdShb7vt]Expressionistic Accoustic[/URL]" playlist on spotify. If you like it please consider checking it out!