Macbeth shows his lust for power first by instantly clinging to the witches’ prophecy that he will be promoted to Thane of Cawdor and king.
The Prince of Cumberland! That is a step(55)
On which I must fall down, or else o'erleap,
For in my way it lies. Stars,...
hide your fires;
Let not light see my black and deep desires: (1:5)
Yet this is not the only time in which his heart has “black and deep desires” during the play. Macbeth is not content to just murder Duncan. He also frames Malcolm and Donalbain, so they flee and are not a threat to him. He murders Banquo and Fleance to ensure that his own sons will be kings.
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown(65)
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding. (3:1)
Thus, Macbeth is content to murder whoever he wants in order to maintain his power now that he has it. He also has spies throughout his kingdom and is quite suspicious and even paranoid.
The worst example of Macbeth’s lust for power is his murder of Macduff’s family. He cannot reach Macduff, because he is in hiding with Malcolm, so he kills his wife, son, and entire household. This is a terrible surge of pointless violence that reinforces to the viewer that he really has lost his mind and has no redeeming qualities.
In addition, this murder spurs Macduff on to be extra vigilant in wanting to kill Macbeth in revenge. Indeed, Malcolm counsels him to turn his grief into rage.
Be this the whetstone of your sword. Let grief(265)
Convert to anger; blunt not the heart, enrage it. (4:3)
In the end, this act of senseless violence may have been Macbeth’s undoing, as it leads directly to his assassination.