This line echoes the Witches' incantation of the end of the first scene of the play:
Fair is foul and foul is fair:
Hover through the fog and filthy air.
So, for Macbeth to echo these words makes him appear to somehow be aligned with the Witches, which casts an evil and foreboding atmosphere around him.
The evil association would simply have come from the social assumption of the general population of Shakespeare's day that witches existed to perform evil deeds (the work of the Devil). Shakespeare didn't need to prove the evil of the witches to his audience, they would simply have assumed it. So, Macbeth's very first impression on the audience links him to the characters that represent evil in the play.
It is also an eerie foreboding that is created when Macbeth utters the words that the Witches have incanted. The suggestion that they have the power and ability to affect the future is implanted in the audiences mind at this moment, adding to the ominous foreboding of the atmosphere of Act One.
Please follow the links below for more on the Witches and how Elizabethans saw them and evil in the play.
As was mentioned in the previous post, at the end of the first scene of the play, the three witches say, "Fair is foul, and foul is fair" (Shakespeare 1.1.12). This motif runs throughout the entire play and essentially means that appearances can be deceiving. The three witches represent evil, mischief, and wickedness. Their evil presence creates an ominous and foreboding atmosphere in the first scene. Macbeth then repeats the witches' statement in his first line in Act One, Scene 3. Macbeth's comment connects him to the three witches and the foreboding atmosphere parallels that of the first scene. This quote suggests to the audience that Macbeth's fate is in the hands of the three witches. Macbeth is immediately associated with the evil witches, and Shakespeare creates a menacing atmosphere with Macbeth's opening lines. As the play progresses, Macbeth is driven by ambition to make a series of fateful decisions that result in his tragic downfall.