Clair may just be an inherently evil person, or, as I suspect, she's acting out late-stage postpartum and stress from losing her mother, and that she's living with an emotionally abusive husband, and then taking it out on Gabrielle in the most horrific way.
I'm reticent to say postpartum because I don't want to equate that with violent tendencies. If it were any other actor playing Clair, I would say she's just an all out nasty person, but Taylor plays her with a flicker of humanity that I feel there's something else going on.