Oh ho, I think I figured out an interesting twist to all this!  D6mn you all for putting this into my head.

So K-Bell is a good guy and Halle is a bad guy.  And in any other movie, Halle would be caught by Kristen at the start and given a chance to redeem herself by joining in.  So why not go the opposite route?  K-Bell is caught by Halle, but the evil organization needs her expertise to deal with a new threat.  Join or die type of deal.  Might add some tension into the dynamic.