arabella is apart of crime syndicate run by mol. They are thieves, They fully grasp what they are doing. khaga planned to lock her up, she decided to try and flee? Try and steal any holy relic from any of the major/minor religions in faerun and you would be slaughtered on the spot, child or not( especially a tiefling). Even a Lawful Good diety like Tyr would have you tried and executed for such a crime.
Why does this take keep popping up? Last time I heard that all the Tieflings are "in a crime syndicate" and therefore irredeemable. Isn't it more that this is their ticket into Baldur's Gate? You know, the "noble refugee turned noble criminal" trope?