The most important things to understand about Macbeth are its essential themes, since these are the reason we still study this play, and consider it a timeless work of literature. In short, Macbeth revolves around two crucial themes. One is human ambition, which, if allowed free rein, can make individuals willing to do horrible things. Indeed, to fulfill his expansive ambition, Macbeth must do these things. Lady Macbeth, who goads her vacillating husband on to commit the first murder of the play, that of Duncan, says as much, in a soliloquy where she questions her husband's will to do what is necessary to fulfill his ambition:
Yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it.
Yet Macbeth and his wife are human, and they experience extreme guilt, itself another important subject, over their actions at various points in the play. Their humanity is made evident in the way it plays out against another major theme at work in the play, the supernatural. It is not until his meeting with the witches early in the play that Macbeth begins to contemplate a future role as King of Scotland. The question of whether the events of the play are driven by malignant supernatural forces or by Macbeth's own all-consuming ambition remains, in part an open one at the end even as it turns out that all of the prophecies of the witches and the apparitions they conjure have come true. The clash between personal forces and those that are beyond our control is one of the essential concerns of Macbeth, and one that modern readers may still find compelling.