As soon as Macbeth hears about the witches' prophecy, his deeply hidden ambition of becoming the king of Scotland is awakened. Shorty after his meeting with the witches, Macbeth learns of his promotion from Ross—he becomes the thane of Cawdor just like the witches predicted—and he becomes aware that one part of the witches' prophecy is true. This leads him to believe that the other part of the prophecy will come true as well:
[Aside] Glamis, and thane of Cawdor!
The greatest is behind.
To ROSS and ANGUS
Thanks for your pains.
It should be clear that the prophecy does prompt Macbeth to fulfill his hidden ambition; yet, it is not the chief reason why he decides to kill King Duncan. Macbeth already harbored treacherous thoughts; however, he has suppressed them for a long time until they are awakened by the witches' prophecy and Lady Macbeth's persuasion. If Macbeth believes the witches are right, he should not take matters into his own hands. He should let the prophecy come true on its own. Instead, though, he goes after his ambitions no matter how evil or wrong they are:
The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires.
With the help of his wife, Macbeth kills King Duncan and all those who could jeopardize his future. He becomes a tyrant, devoid of scruples and compassion.