From the beginning of the play we wonder how much of an influence the witches have on Macbeth. Do they merely nudge him in an evil direction or do they somehow overpower a good man, turning him evil? Many will point to the opening of Act IV as proof that Macbeth is choosing to go down an evil path. He is, after all, seeking out the witches this time for his own personal gain. The witches appraisal of Macbeth as "something wicked" confirms that there is no doubt about it--Macbeth has determined his course. Look at the end of Act III, in scene v for more proof. He uses the metaphor of crossing a river of blood (yuck!) to describe himself as "in blood stepped in so far" that turning back would be just as difficult as continuing on. In other words, his rationale is that since he has committed murder already he might as well continue to do so...Yikes!
The scene with the witches in Act 4.1 of Shakespeare's Macbeth is an amplification of the first two times the witches are featured. Each time they appear in the play their presence and activities grow. In this scene, the witches are preparing a brew, we assume for Macbeth to drink (and that's how it is usually acted out when the play is performed). The witches are presented as even more unnatural, morbid, and grotesque than previously. Their words and their additions to the pot contribute to the blood and animal imagery in the play, as well as the theme of the unnatural. The brew enables Macbeth to see visions.
Shakespeare reserves the use of rhyme in the play for certain situations, and this is one of them. Just before Macbeth arrives, the Second Witch announces:
By the pricking of my thumbs,
Something wicked this way comes.
Whoever knocks. (Act 4.1.44-47)
Of course, the mystique of the witches is added to here, since she appears to have been informed that Macbeth has arrived before he enters, by some sensation she experiences in her thumbs. Macbeth's entry leads to his visions and more predictions by the witches.
The connection between the witches and Macbeth, and the Second Witch's ability to sense him before he enters, should not come as much of a surprise by this point in the play. Macbeth and the witches have been connected since Act 1.3 when Macbeth echoes their fair is foul line.
This is a line from Macbeth. It is spoken by one of the witches. This line comes not from Act I where we first meet the witches, but rather from Act IV, Scene 1. In this scene, the witches are conjuring up evil spirits. Ironically, though, the line is spoken not in reference to the evil spirits they are conjuring. Nor is it spoken about Hecate, who has just left.
Instead, the line is spoken about Macbeth. So, in essence, the witches are calling Macbeth evil, which is certainly a commentary on how low he has sunk.