He might have at one point been a Joe (Napier), but never a Kerr. How very odd.
To be fair, the kind of insanity we see in the Joker is impossible given any sort of realism. This Joker (and the Dark Knight's Joker, for that matter) has been adapted into a setting with a higher realism, and needs to have some actual mental deficits to make sense within the narrative.
I'm in the process of re-reading 100 Bullets, and I feel that Azzarello's The Joker is closer in tone to that then to Dark Knight. Joker using rape as a weapon here? Seems to make sense if you're trying to show the worse human being imaginable. Rape in the regular DCU? Seems horribly off-kilter.