When I was 11 or 12, I found the murderer section in the library. Read a book called "Cannibal Killers" and picked Albert Fish as a favorite because it said he used to light his methane emissions on fire. (I was 12, dangit!) I actually thought his was a fascinating case because he was able to abduct so many children on the basis of his looking like a sweet, old man. Though, his self-mutilation (the pelvis pins) was really... gross. All he wanted were some baby back ribs. (I think he took Swift's "Modest Proposal" to the next level, really.)
For me, the most disturbing serial killer would be Mary Bell. To be so warped at such a young age is a travesty. I hear she's a grandma now, out in the free world.... so I guess there is hope for sociopaths out there... if you catch 'em young enough.
I hear a lot of generic but interesting information on murders from someone I know who works in the area of capitol crimes. Even the basic information (I don't get details) is quite grotesque... which is funny since I enjoy reading about murder, but can't stand films like Hostel or Saw because of the gore.
... before the government re-did their FOIA page, you used to be able to read the FBI profile they did for Scottland Yard on the 100 year anniversary of Jack the Ripper. That was kinda cool to read through. (OoOoh-- found it-- http://vault.fbi.gov/Jack%20the%20Ripper
Ooooh... looks like they even have a page on Gacy http://vault.fbi.gov/John%20Wayne%20Gacy