Why is Macbeth a tragedy?

This quote means that even someone with a good and virtuous nature might give way to a royal command. But my fears cannot make you evil. Angels are nevertheless bright, even though Lucifer, the brightest angel, fell from heaven. Even though evil wants to look good, good still has to look good too.

This passage in Shakespeare’s Macbeth occurs at a point in Act 4.3 in which Malcolm is trying to determine if Macduff is genuinely on his side against Macbeth, or if Macduff is an agent of Macbeth.