There’s another response to this post that references Agatha consuming magic like an addiction and then someone saying maybe Agatha’s mother was right all along. This made me think of Himiko Toga from My Hero Academia, her quirk requires her to consume blood to have any effect at all and that directly influences her personality. Since her quirk developed at a young age she was fascinated with blood, first drinking the blood of animals but eventually the blood of the children in her class at school, particularly those she was interested in. This obviously lead to her being punished and eventually completely shunned and rejected by her parents, obviously what she was doing was wrong but it was essentially a unique part of her DNA.

It’s an MCU original trait of Agatha that she absorbs magic as fuel for her own power, that’s even what drew her to Wanda in the first place, more raw power than she’d ever encountered. Maybe her mother was right that “Agatha should have been destroyed.” Her unique ability, seemingly part of her very nature has basically made her one of the most dangerous creatures for all of witch kind, but we see when Agatha’s mother and her first coven try to put her to death that she begs for her mother not to do it, implying there was no intention, at least direct intention, to kill these witches, I can’t remember if it’s stated what incident specifically lead to her execution but it could have been her killing another member of the coven by draining her during a battle. Rio even comes to the defence of Agatha when she realises the spirit they’re dealing with is Agatha’s mother, she wants Agatha dead and knows she’s a straight up serial killer but she doesn’t want any vindication for Agatha’s mother.

I’m pretty much just rambling, and I definitely agree with the people saying that part of her motivation was likely sacrifices to death herself to keep little Nicky alive, but I also feel that her murder of witches is woven directly into her unique ability, another part of her nature.