Recap: Yana has been pulling all this shit with Ji Bail (including mind controlling him into almost murdering his sister twice, and driving her further into the machinations of the patriarchs) because Yana wanted Ji Bai dead and replaced by her sister Solanya.
Yana's final words: "Yo! You won't forgive for the shit that I've done. And I'm not apologizing -- because I don't regret it one bit... well other than failing to murder you and replace you with my sister. But please do what I want and revive the vampire race anyway!"
At which point Bai Ji kills her, but not to get rid of her, but because she's dead anyway, and Bai Ji is going to preserve her soul to maybe revive her later.
And (IDK about the WN) but the manga is giving zero information about what the heck is going on inside Bai Ji's head when she shows zero inclination towards vengeance.