How does the witches' prophecy of Macbeth's coming greatness acts as a temptation for him?
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:
MACBETH[Aside] Glamis, and thane of Cawdor!
The greatest is behind.
To ROSS and ANGUSThanks 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.
The temptation for power and ruler-ship was too much for Macbeth to overcome. Had the witches not shown Macbeth the prophecy he would gone down a different path.However, once the prophecy was revealed he could not resist and his sinful nature surfaced. As soon as he hear that he would rule, he went to his wife and they planned how to bring about this succession more quickly. He then began to kill off anyone he though might jeopardize this power. The taste for power is a strong temptation for people and it was Macbeth's weakness.