Answer:
Malcolm is trying to convince Macduff about how even the most virtuous person can fall into committing sin. "angels are bright still, though the brightest fell." because we know that Macbeth is introduced as a respected, dignified hero in the beginning, but he fell from his goodness because of his desires, pride, ambition, and Lady Macbeth's persuasion. He is clothed still in grace, being the king of Scotland, but he loses his good nature inside.
An easy translation:
"But Macbeth is. Even someone with a good and virtuous nature might give way to a royal command. But I beg your pardon. My fears can't actually make you evil. Angels are still bright even though Lucifer, the brightest angel, fell from heaven. Even though everything evil wants to look good, good still has to look good too."
Step-by-step explanation:
I hope this helped you in any way.