Oath of Vengeance is also required to help others obtain vengeance, especially if their actions or inactions caused the situation in the first place. It's the 4th tenet of the Oath:
"Restitution. If my foes wreak ruin on the world, it is because I failed to stop them. I must help those harmed by their misdeeds."
Since you failed to stop Kagha from killing Arabella, per the Restitution tenet you are bound to help Komira and Locke enact vengeance for the harm they suffered when their daughter died.
