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.
And what a great syndicate it is, run by a bunch of kids hiding in a cave. Kagha could have easily locked her up without killing her; the druids are all adults (and spellcasters to boot), and Arabella is an unarmed little girl. Kagha chooses to use a viper, when she knows how deadly snakes are and how they act. As a druid she is fully aware of the high risk involved. I doubt a good-aligned deity would be in favor of executing a child in this situation. Simply following the law without mercy is lawful neutral, not good.