Rachel going bad at all is a really, really bad idea. Her becoming Harley Quinn just makes that much more implausible. The Rachel Dawes character from Batman Begins was a friggin rock. Her moral conviction, dedication, and sense of duty and purpose dwarfed Bruce's in comparison, at least at the beginning of the film. It isn't like she had some unstable personality, or rage waiting to be let loose, like Harvey Dent. The idea of her being emotionally fragile enough to try to somehow escape reality and go insane, and falling in love with the Joker is beyond far-fetched, it's just stupid. Harleen Quinzell was taken in by the Joker's twisted pseudo-philosophy because for all her degrees and professionalism, at she was an emotionally fragile, and probably not mentally unstable little girl. Most of, if not all of Batman's villains all display one common characteristic; obsession. Harley Quinn displays this as much any of the Bat-villains, perhaps more than most. In Batman Begins, did you see any obsession in Rachel? She was dedicated to her job, sure, but certainly not obsessed. I like the idea of Bruce having to fight someone he loves, but Rachel Dawes just isn't that character. Besides, isn't that what Catwoman is for?
