WHAT has and made Macbeth certain that mac duff is not his friend?

Macbeth becomes certain that Macduff is not his friend when he learns that Macduff has fled to join forces with Malcolm in England, suggesting that he is plotting against Macbeth. Additionally, Macduff openly expresses his suspicions about Macbeth's rule and leadership, further solidifying Macbeth's belief that Macduff is a threat rather than an ally.