I would like to add to the answer already given. The play opens with the three witches, creating a supernatural atmosphere. The predictions they make tempt Macbeth to committ evil crimes. They introduce the possibility of what Macbeth's future could be if he's willing to be the bad guy. The witches rely on his and Lady Macbeth's ambitions for power to be so great, the two are willing to do whatever is necessary to get what they want. The half-truth prophecies of the witches are successful because they are what Macbeth wants to hear, and they seem to come true right away. The witches don't have the power to set the evil events into motion by themselves, but their power lies in their abilities to get Macbeth to do it. They tempt him, and he gives in. Macbeth is the evil, and the witches provide him with the excuses he needs to carry through with his crimes and achieve his powerful ambitions. So, the witches tempt Macbeth, his ambition for power takes over, and he kills the king. This evil deed creates fear and guilt, and Macbeth feels he has no choice but to kill again.
This is from the enotes theme on Macbeth and the supernatural, if you follow the link, you'll get the whole section:
Beyond the evil that human ambition can manufacture, Macbeth has a super-natural dimension to it; indeed, the play opens with the three witches stirring the plot forward. Even before his encounter with the three witches, Macbeth finds himself in an unnatural dramatic world on the "foul and fair" day of the battle (I.iii.39). Things are not what they seem. After his first conclave with the witches, Macbeth is unable to determine whether the prophecy of the witches bodes "ill" or "good." He then begins to doubt reality itself as he states that "nothing is / But what it is not" (I.iii.141-142). The prophecy, of course, is true in the first sense but not what Macbeth takes it to be in the second. In like manner, the three predictions made to Macbeth in the first scene of Act IV seem to make him invincible; but the "woods" do march and Macbeth is slain by a man not ("naturally") born of woman.