Act 3, Scene 2 demonstrates that Macbeth is selfish as both a ruler and a person.
Once Macbeth has the throne, he is focused only on losing it. He has killed Duncan and been crowned king, but instead of trying to be a king and helping his people, he is paranoid about losing the crown.
We have scotch'd the snake, not kill'd it.(15)
She'll close and be herself, whilst our poor malice
Remains in danger of her former tooth. (p. 45)
He assumes that there is a threat facing him. Worried about maintaining the throne, he makes plans to kill Banquo and spy on his other nobles. Anyone who might be able to stand in the way of his continued kingship is suspect. Macbeth has commented that he fears Banquo because he has heard the prophecies and because his sons seem to be next in line to be king.
Macbeth’s focus on himself, to the detriment of the kingdom, demonstrates that he wanted to be king not because he thought he would be a successful ruler, but because he wanted power. Once he has that power, all he can think about is losing it. At no point in the play does he ever express an interest in the county or its people.