Kabura had a huge crush/love on Haru, which was what made her possible to be turned into a vampire, but that was unrequited love.
Haru got pregnant with a human, she couldn't raise her daughter due to not knowing how (?) so she left her daughter in care of Kabura.
Obviously that hurt her, since a person who she fell in love with ran away, leaving her daughter that closely resembles her, she is jealous as hell but also can't harm Nazuna due to obligation left by Haru and her looking like her lost love.
Think reason why Kabura is stealing/NTR other vampire underlings, is because she also experienced her love being stolen. That's kinda ironic...