Okay, here is my 5cents on this subject. From what I understand, each Beast governs over a specific principle of “human vice” (aka. evil of men). However, when there is more than one candidate for the same Beast slot, each will end up inheriting different, contrasting aspects of said principle. Beast III governs the principle of 快楽 (pleasure), and Kiara herself ended up inheriting the aspect of 愛欲(passion/lust/attachment).
I guess it would be simpler if we just think of this as something like categories and subcategories?