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Ig she’s just confident enough because of the experiences of her past life with him and by the fact that he’s so desperate to get rid of his wife and she was so willing to help. But honestly he could have done away with her and got someone new and just repeated the process. That would definitely have made a bigger statement. Though it probably still wouldn’t have gotten rid of his wife because she’s a psychopath
the way she's so confident is actually hilarious, like she really doesn't think. if the duke got sick of her enough, he can just have her killed. if she gets annoying enough where her death has more pros than cons, it'll happen, the only thing stopping him is that she hasn't gotten to that point YET.
but if she wants to stick to him her whole life, it will eventually come down to assassination, make no mistake. we can assume sally will kick her ass in a socially-acceptable way first, but if that failed... i am not kidding when i say the duke would not hesitate to kill her. she's very silly for not realizing this. like a child, thinking high society can protect her from everything. by all means, push someone who hates you into a corner and see how he reacts.