There is very little said or shown in the play with regard to how Macbeth ruled the people of his kingdom. The best description is to be found in Act 4, Scene 3, where Macduff prompts the newly arrived Ross to speak about general conditions in Scotland with the question, "Stands Scotland where it did?" Ross replies:
Alas, poor country,
Almost afraid to know itself! It cannot
Be call'd our mother, but our grave. Where nothing,
But who knows nothing, is once seen to smile;
Where sighs and groans and shrieks that rend the air,
Are made, not mark'd; where violent sorrow seems
A modern ecstasy. The dead man's knell
Is there scarce ask'd for who, and good men's lives
Expire before the flowers in their caps,
Dying or ere they sicken.
Macbeth is not only ruthless but incompetent.
And in Act 3, Scene 6, Lennox, whom experience has made a lot older and wiser since we first met him when he came with Macduff to wake the murdered King Duncan, implies a great deal in guarded innuendo about how King Macbeth has been behaving in the interim.
Men must not walk too late.
Who cannot want the thought, how monstrous
It was for Malcolm and for Donalbain
To kill their gracious father? Damned fact!
How it did grieve Macbeth! Did he not straight,
In pious rage, the two delinquents tear,
That were the slaves of drink and thralls of sleep?
Was not that nobly done? Ay, and wisely too,
For ’twould have anger'd any heart alive
To hear the men deny't. So that, I say,
He has borne all things well; and I do think
That, had he Duncan's sons under his key—
As, an't please heaven, he shall not—they should find
What ’twere to kill a father; so should Fleance.
But, peace! For from broad words, and ’cause he fail'd
His presence at the tyrant's feast
Lennox gives the strong impression that Macbeth is ruling by fear and that he has spies everywhere. It is dangerous to say what you are really thinking. Lennox knows that Macbeth had Duncan killed and then murdered Duncan's two attendants to keep them from protesting their innocence. Lennox also knows that Macbeth had Banquo murdered and that Macbeth's three murderers also tried to kill Banquo's son Fleance, but Lennox is afraid to say what he thinks in so many explicit words. That is the situation in Scotland under the tyrannical Macbeth. It resembles that of Germany under Hitler or the Soviet Union under Stalin. We get the strong impression that everybody hates Macbeth but everybody is afraid to say so.
Why has Macbeth become such a terrible tyrant? Surely he didn't plan to be a tyrant when he was plotting to become king. Shakespeare had to establish that Macbeth was a tyrant in order to explain and justify the English king's motive for sending an army of ten thousand men to overthrow him. That was a huge undertaking at the time--and its success was not certain. The English king could not interfere in Scottish politics merely because Macbeth had killed Duncan. That sort of thing was going on in Scotland all the time. And the English king could not even be sure Macbeth really had been responsible for Duncan's murder. In fact, he could not even be sure it wasn't Malcolm who was responsible. The English king was concerned about conditions in Scotland because they were affecting him. Thousands were fleeing Scotland and pouring into England. This was spreading the disorder to his own realm, creating crime, strife, food shortages, ruining trade, and creating other problems. And it looked as if this incursion would continue as long as Macbeth remained on the throne. Malcolm could restore order because he was obviously a decent man like his father and because he would be recognized as his father's legitimate successor.