This is about the use and abuse of power. When is power used in the name of good, and when is it used in the name of evil?
One example of power being used for evil is when Lady Macbeth manipulates Macbeth, convincing him to murder the king, even after he'd decided not to. When Macbeth tells his wife that he is no longer interested in killing Duncan, she asks,
Was the hope drunk
Wherein you dressed yourself? Hath it slept since?
And wakes it now, to look so green and pale
At what it did so freely? From this time
Such I account thy love. Art thou afeard
To be the same in thine own act and valor
As thou art in desire? (1.7.39-45)
In other words, Lady Macbeth asks Macbeth if he was drunk when he felt hopeful before (when they made their plans), and she wonders if his hope, perhaps, fell asleep and then woke up "green and pale"—implying he is cowardly and fearful—at what he promised her. She says that this is how she'll think of his love from now on, too: as weak and small. She asks if he's content to want big things but never go after them. She goes on to call him a coward. She implies that he's not a man unless he will commit the murder. Lady Macbeth uses her power over her husband—she clearly knows how to manipulate him effectively—for evil, and she turns him into a monster.
One example of power being used for good is when Malcolm begins to return Scotland to the place it was prior to the corrupt rule of Macbeth. At the end of the play, after Macduff has killed Macbeth, Malcolm takes steps to right the wrongs perpetrated by the old king's regime. He says,
We shall not spend a large expense of time
Before we reckon with your several loves
And make us even with you. My thanes and kinsman
Henceforth be earls, the first that ever Scotland
In such an honor named. (5.8.72-76)
He promises that it will not be long before he repays those nobles and friends who were loyal to him for their service. To begin, Malcolm makes them all earls—a new title in Scotland—to honor them. Next, he promises to bring everyone who has left Scotland in fear for their lives back, to safety. Malcolm likewise promises to find and punish those who supported Macbeth. Like his father, Malcolm seems interested in rewarding those who are loyal and who do him steadfast service and rooting out any disloyal and corrupt parties and getting rid of them. This is a use of his power to, largely, do good.
There are several examples of both good uses of power, and the abuse of power. First, Macbeth himself shows us both the good and the evil. In Act I, he uses his power to lead his army to victory for King Duncan. Because of his bravery in battle, he is rewarded with the title Thane of Cawdor. As the play progresses, Macbeth uses his power to rid himself of potential threats to the throne (Duncan, Banquo, Macduff's family). We find out in Macduff's speech to Malcolm that he has a reign of tyranny, and people fear him. This abuse of power leads to his death.
Duncan, Malcolm, Banquo, and Macduff all serve as good examples of using power. Duncan seems to be a fair and just king, punishing the treacherous and rewarding the loyal. Macolm reveals his desire to restore order to Scotland, using his power to organize a march against Macbeth. Banquo hears the prophecy that his sons will be kings, and chooses to do nothing about it. He lets the natural course of life decide who will be king when, rather than taking that power into his own hands. Macduff shows himself loyal to his country and family above all else, and is rewarded by killing Macbeth.
Negative examples also include Lady Macbeth and the Witches (or Weird Sisters, depending on version). Lady Macbeth coerces her husband to defy the natural order of the universe and kill the king, seizing power that was not meant to be theirs. The witches use their power to meddle in the affairs of Macbeth and Banquo, purposely setting Macbeth down a destructive path. These examples show the consequences of abusing one's power -- despair to themselves and others.