Shakespeare reveals the change in Macbeth's character in two primary ways: 1) Macbeth's actions, and 2) his soliloquies.
Macbeth's actions definitely showcase how the man has morally deteriorated throughout the entire play. When first broached with the idea of power and kingship by the witches, Macbeth has serious reservations about murdering Duncan. As the play progresses, Macbeth's actions grow more violent as he begins to justify his behavior in the name of power. He goes on to hire murderers to kill Banquo and Fleance, and then later sends out henchman who murder MacDuff's entire estate, including his children.
Perhaps the greatest insight into the rapidly changing character of Macbeth is to reflect upon the man's own words. Macbeth's soliloquies help the audience understand the character's internal struggles. For example in Act two, Scene I, Macbeth sees the bloody dagger in the hall:
"Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still" (II.i.41-43).
This speech by Macbeth reveals his paranoia and ever-present guilt for what the crime he is about to commit. However, Macbeth seems almost a different man later in the play after he visits the witches again. Macbeth fears the threat of MacDuff and plans "to give to the edge o’ the sword, His wife, his babes, and all unfortunate souls
That trace him in his line" (IV.i.167-169). Macbeth has truly become a cold-blooded murderer, vowing:
"From this moment
The very firstings of my heart shall be
The firstlings of my hand" (IV.i.163-165).
Macbeth's words and actions both indicate that he has transformed from a man of conscience and ambition to a power-hungry murderer, who will stop at nothing to secure his position as king.