There is a large change in Macbeth's character in Act III, scene ii of Shakespeare's play Macbeth.

Prior to this scene/act, Macbeth was not as forceful in gaining and keeping the crown. Instead, Macbeth was pushed by Lady Macbeth. At one point, Macbeth even tries to push off murdering Duncan (as seen in the following passage).

We will proceed no further in this business:
He hath honour’d me of late; and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (Act I, Scene vii)

Here, Macbeth is not very sure that he wants to murder Duncan (just yet). He has recently gained the title Thane of Cawdor and wishes to see how it "fits" before trying on a new set of clothes (as king).

Later, in Act III, Scene ii, Macbeth is plotting the murder of Banquo. When LAdy Macbeth asks what he is planning, Macbeth replies for Lady Macbeth to "be innocent of the knowledge,
dearest chuck." He has finally taken control over their marriage and the throne.

Basically, in Act III, Macbeth's ambition surpasses his identification of what is right and he becomes a blood-thirsty murderer (in order to keep the crown).