Much of this play is filled with the struggle between light and darkness (symbolizing Macbeth-- he asks for darkness to hide his desires in Act I, and then darkness shrouds the night of the murder). The light in the first two acts is King Duncan, but the struggle went in favor of darkness. This struggle occurs in every act of the play.
Also, in Act V, Scene vii, Macduff enters and says, "If thou [Macbeth] be'st slain and with no stroke of mine,/My wife and children's ghosts will haunt me still" (lines 15 - 16). Macduff can't rest until he gets revenge on the killer of his family, something Malcolm and Fleance (whose family was also killed by Macbeth) didn't say.
Macduff is the hero of the play. He is the light that will soon come to a final climactic battle with the dark (Macbeth). There is also religious meaning to this: God against the devil, Macbeth being the devil (remember how he couldn't say "Amen" in Act II?). This theme has been used in many contemporary stories; it's an epic battle of good vs. evil.
* things are not what they seem
* blind ambition
* power corrupts
* superstition affects human behavior
In Macbeth, ambition conspires with unholy forces to commit evil deeds which, in their turn, generate fear, guilt and still more horrible crimes. Above all, Macbeth is a character study in which not one, but two protagonists (the title character and Lady Macbeth) respond individually and jointly to the psychological burden of their sins. In the course of the play, Macbeth repeatedly misinterprets the guilt that he suffers as being simply a matter of fear. His characteristic way of dealing with his guilt is to face it...