I think there's more to it than that, knowing how charasmatic or manipulative they were does very little for the average Joe, I think some people are just fascinated by individuals who live a life on the complete opposite end of the spectrum to one society wants from us, it's so far removed from normal that characters like this are almost mythical to them.
What's the deal with serial killers seemingly always having quite a following of women, I mean some of them get married behind bars or have hordes of women wanting to keep in touch. There's bad boys then there's these guys at the top of the bad boy pyramid.