I believe that Macbeth's character change is too extreme. From being a loyal subject and essentially a good man, he becomes a terrible tyrant, and in the end he is behaving like a madman. It was not logically necessary for Macbeth to turn into such a hateful tyrant just because he committed a murder to become king. Shakespeare made him a tyrant to justify the military intervention of the English monarch. As Ross describes Scotland in Act 4, Scene 3:

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.

The English king cannot tolerate this state of affairs. He is not interested in Scottish politics and would not raise an army to place Malcolm on the throne unless there were a more urgent reason for the expense and danger. For all the English king knows, Malcolm might indeed have been responsible for his father's murder. But if Scotland is in chaos, it will have a negative impact on England in many ways, including commerce, and an influx of refugees who might be a drain on resources and even behave like hostile invaders.

So Shakespeare has to make Macbeth a consummate tyrant in order to justify the English invasion and the success of Malcolm and Macduff.

I have read somewhere that the Spanish word desperado is derived from the idea of despair. A desperado is a man who has done something so wicked that he is doomed to hell for eternity without any hope of redemption. Since he knows he is going to hell anyway, he has no qualms about committing more sins, since his punishment in eternity could not be any worse. Macbeth seems to have become a sort of desperado with his unforgivable crime of murdering the king, his guest, his friend, his kinsman, his benefactor, his feudal lord. Macbeth himself says in a soliloquy:

For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings! (3.1)

Macbeth feels he has sold his soul to the Devil, the common enemy of man, whom he seems afraid even to name. He is doomed to hell for eternity. No crime he commits after this is of any consequence. He may as well derive what satisfaction he can get out of his kingship while he is still alive. His first step is to eliminate Banquo. He will rule by terror, since he cannot, like "the gracious Duncan," rule by love. The witches will encourage him to be "bloody, bold and resolute." Macbeth says of himself:

I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er.  (3.4)

Concerning Macbeth's character changes in Shakespeare's Macbeth, I'll just elaborate slightly on the first answer above.  When Macbeth first receives the prophecy predicting he will be king and Banquo's heirs will be kings, he is satisfied with the idea of being king.  Banquo's heirs do not concern him at this point.  Once he assassinates Duncan and is crowned king, however, this isn't enough.  Now he wants his heirs to be king.  He asks himself, why should he have taken all this risk just to put Banquo's heirs on the throne?  Unsatisfied with just ruling himself, he plots to kill not only Banquo, but Fleance.  His ambition has grown and become even more menacing. 

At the beginning of the play, Macbeth is a respected general, a devoted husband, and a loyal subject of the king. The first of the witches' prophecies bring out his ambitious nature, but he struggles with killing the king. By attacking his manhood, Lady Macbeth convinces him to committ the first of his evil deeds. Macbeth's evil deed causes him to suffer from fear and guilt, which leads to even more evil crimes. Then Macbeth becomes paranoid, suffering from hallucinations and sleeplessness. He becomes less human as he tries over and over to establish his manhood. His ruthlessness in killing Banquo and Macduff's family shows how perverted his idea of manliness really is.

Macbeth's degeneration is also seen in the collapse of his marital relationship. They are loving and have a mutual respect for one another at first. Lady Macbeth becomes more and more unimportant to her husband after killing Duncan, however. He leaves her out of the plan to kill Banquo, Fleance, and Macduff's family. Macbeth allows the witches to take the place of his wife by allowing them to boost his ego, thinking he cannot be harmed by any man. Macbeth is, of course, mistaken about the witches' prophecies, but this just that he now allows his evil nature to control his actions. By the end, Macbeth has degenerated into evil personified, totally inhumane in his actions.

Macbeth's character changes drastically throughout the course of the play. In the beginning scenes of the first act, Macbeth is shown as a loyal and ruthless fighter under King Duncan. Yet, once the witches meet Macbeth and Banquo and deliver the prophecies, the reader learns that Macbeth has ambition to be king, but a strong belief that "fate will crown" him if it is meant to be. Upon Macbeth's arrival home, his wife is able to convince him that killing the king is the best course of action. They plot and commit the murder together. Thus, the whole of act one demonstrates that Macbeth is a strong warrior with a belief in fate but also great ambitions for power, which Macbeth's domineering wife is able to prey upon.

As the play continues, Macbeth's ambitions but also his guilt giving way to insanity continue to grow, creating a classic tragic storyline. After he is made king, he begins to worry about Banquo's children one day supplanting him and has Banquo killed. When he goes back to the witches to beg more prophecies, he slaughters all of Macduff's family because he knows his fate will become entangled with Macduff's. Yet, Macbeth's desperation to cling to his kingship is coupled with his insanity. Macbeth sees Banquo's ghost at the dinner party in the third act and continues to isolate himself from those around him, including his wife. It is not until the very end of the play, when his demise is certain, that a glimpse of the Macbeth from the first act is partially visible again through his courage in facing his end.

In William Shakespeare’s play Macbeth, the protagonist Macbeth shows a striking moral and emotional change in his personality throughout the course of the play. From a loyal and courageous warrior in the very beginning of the play, he degenerates to a power hungry, ruthless and deceitful murderer. The main cause of this moral weakness is his over-ambitiousness that in turn is triggered by prophesies of the witches. In simple words, Macbeth loses his conscience completely and becomes an anti-hero, evil, insecure, and inhumane personality. His personal relationship with Lady Macbeth also deteriorates. He kills the king, Banquo, Macduff's family and many others brutally. But his character shows a deep shift yet again towards the end of the play. He realizes his fault and choses to fight courageously, knowing and accepting his fate and dying as a hero in the war.