Yeah, i think it's a mixed bag made up of two groups of women: one is the co-dependent type you mention, women with a Florence Nightingale saviour complex taken to a delusional extreme - you'd have to also have quite the superiority complex to think that you could actually 'save' a man like WM or any of the other deeply deranged prison celebrities like Ted Bunny and Ramirez.
The other group of women are psychopaths themselves. Female psychopaths often team up with male psychopaths and take part in their crimes, rapes, murders etc. Even if it is often the male who is the the more overt aggressor, the rapist, for example, the woman often is part of the show and take delight in this. They are known as inverted narcissist/secondary psychopaths. The women who contact imprisoned killers/rapists are merely late to the party and try to get some of the "excitement" of their crimes after the fact and by proxy, by associating with them in any way, even if just by mail. If these women had met the criminal irl, before they were caught, they would also have been the types who would have enabled or helped them in their crimes.
Sick people. Female psychopaths often fly under the radar because they 'hide' behind the male psycho.