Macbeth is a play about ambition run amok. The weird sisters' prophecies spur both Macbeth and Lady Macbeth to try to fulfill their ambitions, but the witches never make Macbeth or his wife do anything. Macbeth and his wife act on their own to fulfill their deepest desires. Macbeth, a good general and, by all accounts before the action of the play, a good man, allows his ambition to overwhelm him and becomes a murdering, paranoid maniac. Lady Macbeth, once she begins to put into actions the once-hidden thoughts of her mind, is crushed by guilt.
Both Macbeth and Lady Macbeth want to be great and powerful, and sacrifice their morals to achieve that goal. By contrasting these two characters with others in the play, such as Banquo, Duncan, and Macduff, who also want to be great leaders but refuse to allow ambition to come before honor, Macbeth shows how naked ambition, freed from any sort of moral or social conscience, ultimately takes over every other characteristic of a person. Unchecked ambition, Macbeth suggests, can never be fulfilled, and therefore quickly grows into a monster that will destroy anyone who gives into it.
From the moment the weird sisters tell Macbeth and Banquo their prophecies, both the characters and the audience are forced to wonder about fate. Is it real? Is action necessary to make it come to pass, or will the prophecy come true no matter what one does? Different characters answer these questions in different ways at different times, and the final answers are ambiguous—as fate always is.
Unlike Banquo, Macbeth acts: he kills Duncan. Macbeth tries to master fate, to make fate conform to exactly what he wants. But, of course, fate doesn't work that way. By trying to master fate once, Macbeth puts himself in the position of having to master fate always. At every instant, he has to struggle against those parts of the witches' prophecies that don't favor him. Ultimately, Macbeth becomes so obsessed with his fate that he becomes delusional: he becomes unable to see the half-truths behind the witches' prophecies. By trying to master fate, he brings himself to ruin.
To call Macbeth a violent play is an understatement. It begins in battle, contains the murder of men, women, and children, and ends not just with a climactic siege but the suicide of Lady Macbeth and the beheading of its main character, Macbeth. In the process of all this bloodshed, Macbeth makes an important point about the nature of violence: every violent act, even those done for selfless reasons, seems to lead inevitably to the next. The violence through which Macbeth takes the throne, as Macbeth himself realizes, opens the way for others to try to take the throne for themselves through violence. So Macbeth must commit more violence, and more violence, until violence is all he has left. As Macbeth himself says after seeing Banquo's ghost, "blood will to blood." Violence leads to violence, a vicious cycle.
In medieval times, it was believed that the health of a country was directly related to the goodness and moral legitimacy of its king. If the King was good and just, then the nation would have good harvests and good weather. If there was political order, then there would be natural order. Macbeth shows this connection between the political and natural world: when Macbeth disrupts the social and political order by murdering Duncan and usurping the throne, nature goes haywire. Incredible storms rage, the earth tremors, animals go insane and eat each other. The unnatural events of the physical world emphasize the horror of Macbeth and Lady Macbeth's acts, and mirrors the warping of their souls by ambition.
Also note the way that different characters talk about nature in the play. Duncan and Malcolm use nature metaphors when they speak of kingship—they see themselves as gardeners and want to make their realm grow and flower. In contrast, Macbeth and Lady Macbeth either try to hide from nature (wishing the stars would disappear) or to use nature to hide their cruel designs (being the serpent hiding beneath the innocent flower). The implication is that Macbeth and Lady Macbeth, once they've given themselves to the extreme selfishness of ambition, have themselves become unnatural.
Over and over again in Macbeth, characters discuss or debate about manhood: Lady Macbeth challenges Macbeth when he decides not to kill Duncan, Banquo refuses to join Macbeth in his plot, Lady Macduff questions Macduff's decision to go to England, and on and on.
Through these challenges, Macbeth questions and examines manhood itself. Does a true man take what he wants no matter what it is, as Lady Macbeth believes? Or does a real man have the strength to restrain his desires, as Banquo believes? All of Macbeth can be seen as a struggle to answer this question about the nature and responsibilities of manhood.