I agree. I think what boiled over was when Barisone stopped being dazzeled by her - when his attention waned, when he realized despite her money/buisness she wasn’t seriosu about learning, or wasn’t going to be the protoge she came to him to be, or else, maybe he always knew that, but he stopped giving her the boosts or admiration she craved because she just wasn’t putting in the work.
When she wasn’t getting the promotion and attention any more, and saw MH getting it, she began to act out, and create the drama which would transend everything, a ‘just’ punishment for abandoning her need for his attention, and “ruin” him. the grander, and more grandios she could make her drama, the better. She would take him down, take his farm, his horses, his family, and do it in a public venu where all would see the “surprises” she had in store for him. She’s a true malignant narcissist. She is mediocre and unacomplished, and failed to bring glory and admiration to herself through him, so destroyed him.