What is the effect of Hecate's monologue in Act 3, scene 5 on the theme of Shakespeare's Macbeth?

Before I begin, I want you to know the difference between
a monologue and a soliloquy. Hecate's speech is a
monologue.

A soliloquy is where a speaker talks aloud
regarding their own thoughts.

A monologue is a long speech
to someon else.

While they seem very similar, I assure you
that Hecate delivers a monologue given she is not reflecting on her own personal
thoughts, but speaking to the three witches.

Hecate is seen
as the supreme witch in Macbeth.  It is during her monologue in Act III, Scene iv, where
her power becomes known.  While the witches have shown Macbeth their prophecy, and it
has come true, Hecate knows even more.  She knows that Macbeth will return to ask the
witches about his future with the crown.

The theme conveyed
in Hecate's monologue is one of fate. In the closing lines of the soliloquy, Hecate
states:

His
hopes 'bove wisdom, grace and fear:/ And you all know security/ Is mortals' chietest
enemy. (III,V,76-78)

Here,
Hecate speaks of the fact that a man secure in his future is one who does not fear.  She
knows that Macbeth will find a false security in the three apparitions prophecies.  She
also knows that his security is false.

That being said,
Hecate's monologue illustrates the theme of fate that is intertwined throughout
Macbeth.  She, only, knows the true fate of what is to come.
Macbeth has seen what he believes to be his fate, or destiny-as provided by the witches'
first prophecy. Unfortunately, fate., itself, is proved to be far more superior than the
trust in fate alone.