The scene that springs to mind immediately is Act IV scene 3, in which Macduff reaches Malcolm in England and the two talk about the state of Scotland and how it has declined since Macbeth killed Duncan and then crowned himself King. There are many possible quotes that you could choose from, so you might like to re-read this scene again. However, one of the most poignant ones is said by Malcolm to Macduff as he gives his assessment of Scotland's state:
I think our country sinks beneath the yoke;
It weeps, it bleeds; and each new day a gash
Is added to her wounds...
Another prominent quote that could be used is Macduff's impassioned response to Malcolm pretending to say that he cannot do anything to help Scotland:
Bleed, bleed, poor country!
Great tyranny, lay thou thy basis sure,
For goodness dare not check thee! wear thou thy wrongs;
The title is affer'd!
Both provide excellent examples of Macbeth's tyranny as seen through the eyes of other characters who have witnessed the changes that have occurred in Scotland first or second-hand.
There are numerous examples throughout the play that demonstrate Macbeth's tyranny. In Act Four, Scene 1, Macbeth meets with the Three Witches, who reveal several apparitions that are designed to purposefully mislead Macbeth. After viewing the apparitions, Macbeth feels overconfident and vows to immediately follow through with his thoughts. Macbeth says,
"The castle of Macduff I will surprise, seize upon Fife, give to th' edge o' th' sword his wife, his babes, and all unfortunate souls that trace him in his line. No boasting like a fool. This deed I’ll do before this purpose cool" (Shakespeare 4.1.157-61).
Macbeth's plan to murder Macduff's innocent family depicts his tyrannical rule. Tyrants are willing to eliminate their competition at any cost, which is exactly what Macbeth does.
Later on in the play, Macbeth's enemies refer to him as a tyrant. In Act Five, Scene 2, Menteith is speaking to Caithness and asks what Macbeth is doing. However, Menteith does not call Macbeth by his name, and instead asks, "What does the tyrant?" (Shakespeare 5.2.12) Caithness responds by saying,
"Some say he’s mad, others that lesser hate him do call it valiant fury. But, for certain, he cannot buckle his distempered cause within the belt of rule" (Shakespeare 5.2.13-16).
Caithness's description of Macbeth portrays an out-of-control tyrant who has lost his mind. Throughout the play, Macbeth becomes corrupted by ambition and becomes a ruthless tyrant.