who tao's feed

I've seen the spoilers and might drop this but not now. As the spoiler said the duke will kill his wife's brother but they just said that he will kill him, so what is the reason? They didn't tell the reason so I can't really judge for now. Just thinking that the duke still kill his brother-in-law despite knowing that his wife might be sad and resent him, he still did it. So, my point is there might be a valid reason(there must be because if not I will be the one to kill the duke). Her brother is just too precious to lose, he is the only family the fl had(except his husband and son). Her brother is kind and caring so I wish they will change that part