Each character in Macbeth has to either fight or give in to the evil. Because evil is contrary to human nature, Macbeth and Lady Macbeth have to go against their own conscience in order to murder Duncan. When the witches predict that Macbeth will become Thane of Cawdor and later, King of Sc.
For Shakespeare, human nature has the potential for both good and evil, depending upon the decisions we make. Macbeth is ambitious, true: but not only is ambition, in the sense of a desire for the just approbation of one's fellow men a good quality, but Macbeth is not so ambitious that nothing else matters. His ambition for approbation sets.