In Macbeth, how is the relationship between cruelty and masculinity related to the three witches?

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.