It's too bad there isn't a tag for "Surprise Twist Endings". This manga sure has one of those. IF you know someone that always puts down manga for their predictable endings, show them this one and see if they can predict this ending.
I'm assuming the last girl was his fantasy. Except he didn't know her past which created a hidden twisted self. So when he overcame the girl's self control with the psychic rings, the twisted self came out.