The witches' proclamation illustrates one of the most prominent themes in Macbeth - appearance versus reality. We can infer from the play that what appears to be fair is actually foul. This can be best explained if we take a look at our tragic hero, Macbeth.
At the beginning of the play, Macbeth is depicted as a valiant and loyal warrior, defending Scotland from the Norwegians and the Scottish traitors. He appears to be as one of the most reliable and trustworthy king Duncan's subjects. Nevertheless, after his surreptitious ambition of becoming the king of Scotland is awakened, we see him as disloyal, treacherous, and unscrupulous. Despite the fact that Duncan and Macbeth are related and that Macbeth is Duncan's subject, Macbeth sees him as the greatest impediment to the realization of his ambition and, therefore, kills him and becomes the king himself. This is the moment when Macbeth's tragic downfall begins.
Even the witches describe Macbeth as "wicked" before he appears in front of them to ask them about his future:
By the pricking of my thumbs,
Something wicked this way comes.
Although we may first describe the witches as the agents of evil, we realize that Macbeth is, in fact, the one who awakens evil within him and who resorts to committing dreadful deeds. By murdering innocent people in order to establish himself as the untouchable monarch, he rejects goodness and embraces wickedness. This eventually costs him his life.