Macbeth usurps kingship by killing the good old king Duncan. The Prince of Cumberland and the legitimate heir to the throne of Scotland, Malcolm, escapes to England thus allowing Macbeth to wear the robe of the giant king in spite of being the 'dwarfish thief'. Not only individuals like Banquo and Macduff, but also the Scottish people generally suffer under the unlawful and tyrannical rule of Macbeth. Ever since the failed coronation Banquet that exposes the killer and usurper king, he is consistently referred to as the 'tyrant'.
The more Macbeth is haunted by his guilt and fear, the more the more unrelentingly cruel and frightening he becomes. He unleashes a reign of terror in Scotland in order to ensure his safety and security. A vivid picture of the whole Scotland suffering terribly under Macbeth's tyranny can be had from these words spoken by Macduff to Malcolm in act 4 sc.3:
.....................................................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 yell'd out
Like syllable of dolour.
The lesson seems not too difficult to derive--absolute power obtained through unlawful mode has to be held in possession by unleashing mindless tyranny.