What two things convince Malcolm of Macduff's loyalty and sincerity in Macbeth?
Malcolm is convinced of Macduff’s loyalty when he remains loyal despite Malcolm’s protestations that he would not be a good king, and is at the same time passionately ready to fight for Scotland himself.
Malcolm is worried. He thinks that he can trust Macduff, but after all, Macbeth seemed trustworthy too at first. Look how that turned out! Macbeth murdered the king and turned into a bloody tyrant. Macduff swears to Malcolm that he’s not treacherous, and Malcolm reminds him that Macbeth is.
Angels are bright still, though the brightest fell.(25)
Though all things foul would wear the brows of grace,
Yet grace must still look so. (Act 3, Scene 3)
Malcolm may be young, but he is no fool. He decides to tempt Macduff. He begins by explaining to him that he is not going to be a good king at all, having his way with all of the young women and spending the country’s money as if it were his own, hoping Macduff will jump in and prove his treachery by trying to take over.
Macduff is as honorable though, as Macbeth is not. He really is ready to fight with Malcolm and follow him, and he really is noble. He has no thoughts of betraying Malcolm or anyone else. He makes impassioned speeches full of raw emotion that show how much he cares about his country and how much it pains him to see how Macbeth has betrayed them.
Indeed, it is Macduff’s stirring speeches about how much he loves Scotland and how read he is to fight for it, and fight Macbeth, that convince Malcolm that he is the man for the job. Malcolm decides that he going to go to war against Macbeth with Macduff by his side as a loyal lieutenant.
Macduff, this noble passion,
Child of integrity, hath from my soul(130)
Wiped the black scruples, reconciled my thoughts
To thy good truth and honor.
Of course, Malcolm has right on his side as he fights this battle. He is intelligent and noble, and Macduff is brave and passionate. Together, they will win.
Shakespeare carefully juxtaposes the innocence of Macduff’s raw passion with the violence of Macbeth’s bloodlust. While Macduff seeks only revenge, Macbeth seeks only power. Here in this scene we see that Macduff hates Macbeth for what he has done, and for what he has done to their country, but he does not seek power. He is as noble as Macbeth is hollow.