Macbeth had strong misgivings about murdering King Duncan even before he committed the treacherous and treasonous act. He knew he was doing wrong and making a terrible mistake. He loved Duncan, and Duncan loved him. Throughout the rest of the play he is tormented by guilt, as almost anybody would be who had committed a cold-blooded murder. (King Claudius inHamletis tormented by the same guilt feelings.) So gnawing guilt is one of the reasons for Macbeth's character change.
Macbeth may have been a good follower, but he is not a good king, and he knows that many of his subjects hate him. He uses force to rule, and this only creates resistance and desertion. Unlike Duncan, who was kind and inspired loyalty, Macbeth becomes more and more tyrannical. He is a soldier. It is natural for him to act like a soldier. He is only comfortable when he is fighting.
Macbeth is not a thinker. He relies on his wife to tell him what to do. Later he relies on the Weird Sisters for advice. A man like this does not make a good ruler. His kingdom is being ruined by misgovernment. He is becoming desperate because he doesn't know what to do about it. The news of the approaching English army only makes him more desperate.
There is a significant moment in Act 5, Scene 3 when he says to Seyton: "I'll fight till from my bones my flesh be hacked. Give me my armor." Seyton replies: "'Tis not needed yet.'" Macbeth says: "I'll put it on." He is more comfortable as a soldier than as a king. He knows how to fight but not how to govern. He is not a good administrator or leader, and he knows it.
In Act 5, Scene 2, Angus wisely summarizes Macbeth's state of mind:
Now does he feel
His secret murders sticking on his hands.
Now minutely revolts upbraid his faith-breach.
Those he commands move only in command,
Nothing in love. Now does he feel his title
Hang loose about him, like a giant's robe
Upon a dwarfish thief.
What really caused Macbeth's change during the course of the play was that he made a terrible mistake in thinking he could be king.