In the beginning, Macbeth is showing himself strong as a warrior. He has proved to be fierce on the battlefield. He doesn't seem to be power thirsty in the beginning.
After he meets with the witches, they prophesy that he will be the king of Scotland. Then, Macbeth begins changing. He begins to visualize himself as king of Scotland.
When King Duncan mentions that Malcolm will be Prince of Cumberland, Macbeth states to himself that he will need to jump over Malcolm or else fall on that step. He mentions that his thoughts are black and deep:MACBETH:
[Aside.] The Prince of Cumberland!I must fall down on that Step, or jump over it,For it’s in my way. Stars, put out your light!Don’t let anyone see my black and deep desires.It could happen in an instant Yet let that go,That thing that the eye is afraid to see when it is done.Here we see that Macbeth is beginning to plan for greater things than he has already been given. King Duncan has made him Thane of Cawdor, just as the witches prophesied, but this is not going to be enough for Macbeth. He has the position of king of Scotland on his mind.
The next time we hear from Macbeth, he is planning to murder King Duncan. When Lady Macbeth plants thoughts of murdering King Duncan, Macbeth states that they will discuss it more so:
We will speak further.The next thing Macbeth is planning is cold-blooded murder:
I am settled, and bend up(90) Each corporal agent to this terrible feat. Away, and mock the time with fairest show: False face must hide what the false heart doth know.