Kagha is a shadow druid like Faldorn (neutral evil).
As for the stealing a sacred relic, I disagree that all faerun religions would kill the ofender. Lawful good might if they are more lawful than good. But that would not be good even if the thief is an adult. It would be neutral being generous. After all, death penalty for theft is against human rights. It might be lawful, but still moraly wrong.
And I think that is exactly the point. Arabella is a thief, she is not inocent, but she doesn't deserve to be killed anyway. She deserves a more balanced punishment, not death. And a Tyr palladin would know that. Punish should fit the crime otherwise there is no justice, just revenge.