What is a quote that clearly shows that Macduff does not trust Macbeth?
In Shakespeare's Macbeth Act 4.3, Macduff is with Malcolm in England under English protection. Macduff is there to align himself with Malcolm and unite against Macbeth. His first words to Malcolm are:
Let us rather
Hold fast the mortal [deadly] sword, and like good men
Bestride our down-fall'n birthdom [protectively stand over our native land]. Each new morn
New widows howl, new orphans cry, new sorrows
Strike heaven on the face, that it resounds
As if it felt with Scotland and yelled out
Like syllable of dolor [similar cry of anguish]. (2-8)
Macbeth is talking Malcolm into uniting to fight against Macbeth. These lines clearly show what Macduff thinks of Macbeth and his reign as king of Scotland. Widows howl and orphans cry: men are being killed. And the heavens even cry out, expressing the Elizabethan view that the heavens reflect the state of human affairs.
Macduff is clear here, yet, in a play in which fair is foul and foul is fair, Malcolm isn't convinced. He spends the next few minutes testing Macduff, and finds out that Macduff is speaking honestly when he condemns Macbeth in the lines quoted above.
And, of course, by the end of the scene news arrives in England that Macduff's family has been slain. There can be no doubt then that Macduff is out to kill Macbeth.