According to Mercury himself, “Killer Queen” is about a “high class” prostitute. The main character of this song is about the “Killer Queen”. There are three main characteristics that she possesses. One is that she enjoys a high-class, pricey lifestyle which includes interacting with top-level powerbrokers, and she actually seems quite smug about it. Second, she is very seductive. Finally, third and perhaps most importantly is that the band perceives her as being lethally dangerous.
More than anything, the overall purpose of this song appears to be to function as a warning to the listener against getting involved with the Killer Queen rather than actually criticizing her lifestyle or meaninglessly documenting her lethalness.