Lurid headlines regarding UK schoolgirls fleeing to Syria like "brainwashed in their bedrooms" intuitively appeal to contagion models of why people go bad. We saw this during the Red Scare, where exposure to communist ideology was tantamount to contamination which had to be purged from organizations. However, despite intuitive appeals, it's clearly wrong, as a contagion model clearly does not explain why more people aren't affected.
Given the objectively small numbers, a more useful model to consider would be why women fall for serial killers. Although videos of serial killers in action and their aftermath are rare, photographic evidence and accounts are readily available on the Internet as well. That subject isn't very well studied, but there's no shortage of more plausible speculation as to why women may fall for men who kill. Schwyzer's proposed mechanism of envy while trapped in a highly demanding situation seems to fit, particularly given that the 3 schoolgirls were all reportedly straight A students and therefore likely under high pressure to perform:
the fascination may be less about attraction than about a strange kind of envy of the shows' sociopathic villains. How many bright, talented, acutely sensitive young women have occasionally fantasized about having an internal "mute button" that could silence the judging, nagging, needy voices of all around them?