Before he hears the witches' prophecy, Macbeth was pretty content with his life. Now he's having some pretty naughty thoughts—thoughts about killing his king. He's not exactly excited about the prospect, but he's also not exactly exploring other options.
Turns out, murder is easier than he thought it would be. Macbeth frames Duncan's guards with his wife's help, casts suspicion on Duncan's sons, and takes the crown for himself. Victory complete. Right?
Actually, it turns out that, if you become king through treachery, you end up suspecting everyone else of treachery. Macbeth plots (and carries out) a few more murders, but he's still feeling uneasy.
Talk about a nightmare: the murdered Banquo shows up to cast a little gloom over Macbeth's banquet by throwing him into a raving fit. That same night, Macbeth has received intelligence that Macduff, Thane of Fife, has gone to England to gather forces with Malcolm and Siward. Yep, things are falling apart pretty quickly.
Turns out, you can't trust a witch. Lady Macbeth commits suicide; Birnam Wood actually does come to Dunsinane; and Macduff was delivered via C-section rather than "born," which means that his sword has Macbeth's name on it. The one good thing? He leaves the play a warrior —just as he entered it.