Final answer:
The action that shows Macbeth's further descent into madness in Act IV is when he pointlessly orders the murder of Macduff's family, revealing his paranoia and loss of morals.
Step-by-step explanation:
The action in Act IV that illustrates Macbeth's further descent into corrupt madness is B.) He pointlessly has Macduff's family murdered. This act of violence exemplifies his increasing paranoia and the loss of his moral compass. After being unable to murder Macduff himself, Macbeth targets Macduff's innocent family, showcasing a sense of desperation and a willingness to commit atrocities for the sake of retaining power. Macbeth's descent into madness is marked by his unfounded fear and his loss of humanity, cementing his transformation from a noble Thane into a tyrannical ruler tormented by his own actions.