Robin Gecht certainly worked his way up the serial killer ranks. Initially, Gecht learned the trade, so to speak, by working for John Wayne Gacy, but he eventually amassed his own group of homicidal colleagues (Edward Spreitzer, Andrew Kokoraleis, and Thomas Kokoraleis) and made a name for himself.
In 1981 and 1982, on the streets of Chicago, Gecht and his buddies drove around abducting prostitutes, forcing them into their van, and bringing them back to Gecht's apartment where they'd murder (or, rather, sacrifice) the women in gruesome Satanic rituals. Per the killers' own claims, they would cut off one of the prostitute's breasts and eat it as Gecht would read aloud from Church of Satan founder Anton LaVey's 1969 book The Satanic Bible.
And that's not all. Once they finished eating, they would take turns raping the area where the breast had previously been, before masturbating onto the breast and consuming it in little pieces.