Well, from studying serial killers, I've always believed that serial killers who were "destined" to become so at a young age can't "turn off" the desire to kill. Most of them, anyway. They'll continue to kill until they are behind bars, and if they are eventually freed, they will kill again. This isn't a universal rule, as serial killers have stopped killing before, but there are plenty of instances of killers imprisoned, sometimes for other crimes, released years later, and they go right back to killing. So, of course there are exceptions, but I do think those truly sick and evil people battle that urge their entire lives. Maybe she is one of them, but as far as we know she hasn't committed another heinous crime since her release. I think it was a perfect storm of two people meeting and they kinda yin and yanged so to speak. His deviant and murderous desires perfectly complimented her obsessive, haywire, berserk personality. Remove him from the equation and I think she's just your crazy girlfriend with emotional issues, but without him, I doubt she commits a murder in her entire life. He was sadistic before he met her, so I think he may have always turned out that way even if he didn't meet her. He showed signs of sadistic deviant sexual behavior early, and I think he naturally would have progressed into a killer eventually.
That's just my personal theory. I think she had a very volatile and possibly somewhat psychotic personality, but I don;t think she would have been a killer had she not met Bernardo.
