Act 5, Scene 8: 'Turn, hell-hound, turn'
Macduff compares Macbeth to a creature of hell, which echos his language in Act 4 where he refers to him as a 'Hell-kite'.
Macduff sees Macbeth for who he truly is- evil.
The use of imperative contrasts with Macbeth's in earlier scenes and shows that Macduff is now the one in control.