In the opening of Act IV, the witches are focused on creating potions. Here is what they say when they speak of the ingredients:
Finger of birth-strangled babe
Ditch-delivered by a drab,
Make the gruel thick and slab.
Add thereto a tiger’s chaudron,
For the ingredients of our cauldron.
In what is being made, the witches have established a sense of the wicked. Their ingredients represent elements in the world that embody repugnance. Limbs of babies delivered by prostitutes, internal organs of animals, and unnatural deaths are wicked elements. This serves as a prelude to the human embodiment of wickedness when they remark that Macbeth has approached. It is Shakespeare's genius to have the witches--sources of wickedness--recognize its human form in Macbeth. Seen another way, for the witches to remark that "something wicked this way comes" when Macbeth approaches helps to give some insight to the audience as to how depraved Macbeth is and how much further down the rabbit hole of moral depravity he will descend. The witches' statement is valid given Macbeth's moral slide so far in the play; it also foreshadows how much further Macbeth will fall into wickedness.
I think that a case can be made that some qualities of the Macduffs make them the opposite of the Macbeths. One stark difference is in their reflective capacity. Macduff is more reflective and insightful than Macbeth is. Even when Macbeth is struggling with whether to kill Duncan, his thoughts are geared toward himself. However, Macduff is able to think of more than just himself. We can see this both in the early and later parts of the drama. Whether Macduff speaks about the shifting condition of political rule ("Well, may you see things well done there: adieu, / Lest our old robes dit easier than our new" (II.iv.37-8) or when he speaks about pain as the only universal quality for all human beings, Macduff possesses a reflective capacity that Macbeth lacks:
Each new morn
New widows howl, new orphans cry, new sorrows
Strike heaven on the face, that it resounds
As if it felt with Scotland and yelled out
Like syllable of dolor.
I am not entirely comfortable seeing Macbeth and Macduff as complete opposites. For example, they are similar in that they both leave their wives in pursuit of political ends. Even though Macduff's flight might be more honorable than Macbeth's actions, Lady Macduff speaks eloquently in criticizing her husband's "wisdom" to leave her and their child behind. In the same way, Macbeth emotionally abandons his wife in pursuit of his political ends. Shakespeare might be suggesting that modern individuals, in pursuit of professional aims, run a risk of alienating those we should be caring for the most.
There are, however, instances where the women are opposites. Lady Macbeth, coveting power and encouraging her husband to commit a foul deed, is diametrically opposed to the Lady Macduff we see in Act IV. Similar to her husband, Lady Macduff possesses a greater capacity for reflection than her counterpart:
Whither should I fly?
I have done no harm. But I remember now
I am in this earthly world, where to do harm
Is often laudable, to do good sometime
Accounted dangerous folly. Why then, alas,
Do I put up that womanly defense,
To say I have done no harm?
Like her husband, Lady Macduff is able to speak from a wider perspective that encompasses all human beings. Even though Lady Macbeth does gain some reflective capacity, it is limited to herself. Again, I think that both women are not pure opposites because of the estrangement that they experience. Both women experience an abandonment from their husbands in a time of need. Lady Macbeth needs her husband emotionally and on a spiritual level. Lady Macduff needs her husband physically--for protection against the murderers that Macbeth sends against her and her child.
The sleepwalking scene in Act V, Scene 1 is a testament to how emotionally weakened Lady Macbeth is. When she sleepwalks, it is clear that the murders have taken a toll on her emotional condition: "Here’s the smell of the blood still. All the perfumes of Arabia will not sweeten this little hand. Oh, Oh, Oh!" It is an instant where she needs psychological help. She needs her husband. The sleepwalking scene reflects how alone and lonely Lady Macbeth actually is. Up to this point, the extent of her estrangement had not been fully known. The sleepwalking scene makes it clear that guilt and regret have overtaken her. She is a shell of what she used to be. When the Doctor suggests that her mind is "infected," it is clear that Macbeth needs to be there more for her. The pursuit of his own political ends has taken a fatal toll on his wife. Her transformation from what she was to what she is represents one of the most powerful elements in the drama.
Different people will experience different emotions at the end of the drama. I am not sure I feel sorry for Macbeth, as much as I feel sorry for all of humanity. Macbeth is a great work in part because it functions as a cautionary tale. In a world where people still cling to external reality more than anything else, Macbeth's suffering can often reflect our own. He is a reminder that shallow pursuits end up creating a shallow human being. It is this regard where my sorrow at the end of the drama is not as much for Macbeth the character, but for all of us.