The first thing worth thinking about is whether Macbeth, over the course of William Shakespeare's play Macbeth, actually does change character, or whether the monstrous character that is gradually revealed is one he possessed all along and which gradually becomes more apparent to us over the course of the play.
The key event in the play that begins the change is the encounter with the weird sisters on the blasted heath. In this scene, Macbeth evolves from merely be ambitious to be outstanding in service of his king to wanting to be king. We may regard this as a change in degree of ambition.
The next change is the murder of Duncan. He seems strongly influenced by Lady Macbeth in this. One can argue that once he has done the evil deed of killing Duncan, he loses all inhibitions that might have held his evil impulses in check, and not only becomes more violent but loses his capacity for self-restraint.
In Act One, Scene 3, Macbeth is told by the Three Witches that he will become the Thane of Cawdor and rule as king one day. The thought of one day becoming king intrigues Macbeth, and he begins to contemplate the witches' prophecy. Macbeth has not thought about murdering King Duncan until this moment; before this, as far as we know, Macbeth has been a loyal subject of King Duncan and is willing to risk his life in battle for his country.
By Act Three, Scene 1, Macbeth has not only committed regicide but begins ruling his country like a tyrant. Macbeth sends assassins to kill Banquo and his son in order to challenge fate and secure his royal legacy. Macbeth does not want Banquo's descendants ruling as kings. Macbeth's decision to murder his close friend and innocent son reveals his bloodlust and corrupt nature. In just two acts, Macbeth goes from being a loyal subject of King Duncan to a tyrannical murderer obsessed with maintaining his power at all costs.