Once he hears the witches' prophecy that he will be King of Scotland, Macbeth begins to think about what actually achieving this position will entail. It is evident that he realizes almost immediately that he will have to commit murder to become King, as he implies in an aside after Malcolm has been made Prince of Cumberland, the title given the heir to the throne:
Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see.
Urged on by his wife, these "black and deep desires" come to fruition in Act II, as Macbeth soon murders Duncan while hosting him in his own castle. He then has Banquo, a man who is portrayed as his good friend in Act I (he accompanied Macbeth when they heard the witches' prophecy) murdered, though his son Fleance escapes. Just before he kills Duncan, Macbeth comments on his motive, which is very simply ambition:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other...
Macbeth is indeed motivated to commit horrible deeds by ambition, but by Act IV, he has quickly degenerated into paranoia, having Macduff's family murdered (though Macduff himself escapes.) He has come to view Macduff as a major rival, his suspicions sparked by Macduff's absence at a banquet. By the end of the play he is delusional and overconfident, having put all of his faith in the witches' prophecies. He is clearly willing to stop at nothing to achieve and maintain power, and the horrible acts he commits each necessitate further horrible acts.