Just because she did not follow him as he demanded of his duchess. Leaves one asking if he'd ever told his wife how he felt, did he ever indicate that he ended up feeling so murderous due to her actions? One would hope that as a married couple and all that the Duke would speak about her acts of insolence with his Duchess. The duke attributes to his social status the inability to express his preferences to his wife. Even if she would tolerate a criticism or guidance. Not unexpectedly, a man with such arrogance will not lower himself in order to say her his will. She should agree with him all the time, he assumes. The duke believes she should be able to read his mind