William Shakespeare's Macbeth follows the typical characteristics defined by Aristotle (in Poetics). One of these characteristics is defined as the protagonist's possession of hamartia (tragic flaw). In Macbeth's case, his hamartia is his lofty ambition.
Macbeth's ambitious nature proves to be far too strong for him to control. Not only does he murder his king (King Duncan) in order to take the throne for himself, he plots murder against Banquo and his sons (given the prophecy denoted Banquo's sons as possessing the throne) and murders Macduff's family.
Therefore, in order to keep the throne as his own, Macbeth's ambition far outweighs his moral nature and he murders without true thought (with the exception of the murder of Duncan--which he contemplates completely).
As for the effect his ambition has upon his kingdom, many of his followers (the followers and supporters of Duncan) fail to truly accept him as king. Not only does Banquo question his actions, "I fear thou play'dst most foully for't," he understands Macbeth's trust in the prophecies. Banquo knows that he and his sons may be in danger.
Outside of Banquo, other members of Macbeth's court are not comfortable with Macbeth's reign. In a conversation between Ross and Macduff, Macduff admits that he is not happy with Macbeth sitting on the throne.
Lest our old robes sit easier than our new!
In the end, the murder of Macbeth proves to be the most solid support that the people of his kingdom do not accept him as their king.