How does Macbeth explore the nature of evil?

Macbeth is perhaps Shakespeare's greatest exploration of the problem of evil. The implications about evil that come out of the play, according to me, are---

1. Evil is positioned both within and without. The witches are objective figures alright but Macbeth's first utterance in act 1, scene 3 suggests that he has an uncanny proximity of thought with the witches. When he says that he has never seen such a fair and a foul day, he seems to echo the maxim of the witches--"Fair is foul foul is fair". This is before his encounter with the witches. This is an unconscious figuration of the evil as repressed in human psyche which is surfaced through the temptations of objective evil. So, evil is not just supernatural, but human too.

2. Shakespeare's masterful implication lies in the way he relates evil to language. There is an evil of equivocation, that is championed by the witches. Their speech is full of riddles, contradictions and menacing redundancies that give false support to the ego and create the complacency of invincibility. The prophecies tell Macbeth that no one born of the woman can kill him and he is made to think that he is immortal. At the end, it is revealed that Macduff, his executioner, is untimely ripped from his mother's womb. This is how an apparently absurd conditioning comes true to menace Macbeth.

3. There is justice in the name of retribution as a response to all evil and it will always happen in course of the life where the sin is committed. This is what Macbeth's soliloquy spells out at the end of act 1.

4. Evil is above all an act against the natural, the norm of the macrocosm as well as the microcosm. Evil is imposition. It is an artifice, a garb of sorts. Evil drives Macbeth into murdering the innocence of sleep while Lady Macbeth loses her innate gender identity, her feminine qualities in charge of evil.

5. As the fair-foul maxim goes, the play presents to us evil in all its grey shades--the mutually interchangeable nature of good and evil as a critique of the morality play simplicity of the divide.

6. There is ethical thought even in the evil-doing subject, as we see its course in Macbeth, especially the Macbeth before the murder. The stoic ethics comes back in the 'tomorrow and tomorrow'  speech with all its admitting dignity of tragic suffering. Lady Macbeth in her sleep-walking scene, once again returns to disturbing ethical questions about what really happened.