I don’t remember that ever being explained, it may have been a one off line from Valerie, but I don’t think she mentioned Kai at all. Even if it was I’m definitely not sold because I feel as though it should have come up more during S8 when they were siphoning Caroline on accident, even that speech Bonnie gives them, it’s not about how much they siphon it’s just about control overall.

In reference to Caroline, she dead, literally, the only thing keeping her alive was magic so being pregnant with babies who can siphon magic and have no control or probably idea what is happening literally means death for her. That’s how they got magic.

I doubt that siphoning Mama Parker would kill her because magic isn’t what was keeping her alive plus Jo was there as well so he wouldn’t even be solely siphoning her. Them hating Kai was probably just their own prejudice because if I recall, the heretics were apart of the Gemini coven before being exiled due to what they were.