The relationship between cruelty and masculinity is most fully explained in Lady Macbeth's soliloquy in Act I, Scene v, when she summons the spirits to "unsex me here" and to fill her with "direst cruelty." The inference is clear: Cruelty is a masculine trait not found in women.
Although it is Lady Macbeth who develops the idea, the relationship between cruelty and masculinity is first introduced in the play through Shakespeare's characterization of the three witches. Scene iii of Act I opens with the weird sisters alone on the stage engaging in a conversation that reveals their malevolence and cruelty. The First Witch explains with relish the terrible torment she will inflict upon the husband of the woman who has offended her; her two companions spur her on and offer their services to help her in torturing him. To emphasize the cruelty of the witches, the First Witch carries with her "a pilot's thumb, / Wracked as homeward he did come."
Having established the witches as incredibly cruel, Shakespeare then portrays them as being masculine in nature. Banquo describes them when he and Macbeth first meet them on the heath:
You should be women,
And yet your beards forbid me to interpret
That you are so.
The relationship between cruelty and masculinity, established early in the play, establishes the motif later expanded upon in Lady Macbeth's speech.
The three witches in "Macbeth" motivate much of the action of the play since Macbeth becomes driven by their predictions. These witches appear masculine as they are described as wearing beards. In Act I, Scene iii, Banquo says to the witches who turn "fair to foul, and foul to fair,"
....You should be women,/And yet your beards forbid me to interpret/That you are so (45-48)
Later, in Act IV, Scene i, the witches express their delight in cruelty as they make a thick gruel from body parts of non-Christians such as Turks and Tartans. One of the witches rejoices in the appearance of the evil Macbeth; she says she can feel it
By the pricking of my thumbs/Something wicked this way comes (44-45).
That the witches' masculinity has an evil overtone is evidenced in the acceptance of the predictions of the "weird sisters" by Lady Macbeth who is led to an unnatural phantasmoagoric realm of witchcraft in which she transforms herself into an unnatural desexualized evil spirit so that she can motivate her husband in his cruel designs for power as well as commit her own acts of cruelty.