Power is used appropriately by Malcolm, but abused by Macbeth. The battle in Act 5 is a perfect example of each.
An essay on power and abuse of power should focus on incidents of using power appropriate, and using it inappropriately. All rulers have power. Rulers who use their power appropriately are just and honest. They have integrity. Macbeth is not one of these rulers. He got his power dishonestly, by killing the proper king. Once king, he was a tyrant. He did not care about his people, and did not rule justly.
Macbeth is called a tyrant repeatedly. Malcolm comments that he loved Macbeth once, and used to think he was honest.
This tyrant, whose sole name blisters our tongues,
Was once thought honest. (Act 3, Scene 3)
Ultimately, Macbeth does abuse his power. A perfect example is how he treats his soldiers. He barricades himself in his castle and expects them to fight to the death defending him. He threatens to punish them just for giving him news that he doesn’t want.
Go prick thy face and over-red thy fear,
Thou lily-liver'd boy. What soldiers, patch?
Death of thy soul! (Act 5, Scene 3)
This is a clear abuse of power. If he had gained his power justly, he could use it justly in fighting a proper war. Instead, he expects his men to give up their lives for him just because he stole the throne.
Malcolm also brings an army to this battle, but he inspires his people instead of threatening them. He tells Macduff that he does not want to take the throne. It is rightfully his. It is his duty to fight to protect his kingdom.
Cousins, I hope the days are near at hand
That chambers will be safe. (Act 5, Scene 5)
People follow Macbeth because they are forced to, or because they are loyal to the kingdom. He abuses his power. They follow Malcolm because they love their country, and they refuse to allow Macbeth to defile it.