Why do you think Shakespeare chose to portray Macbeth as so conscience-stricken by his deed? How would the story be different if Macbeth’s actions caused him no moral suffering?
If Macbeth's conscience had caused him no moral suffering, then he would be an out-and-out villain, somewhat like Shakespeare's Richard III. However, Shakespeare wanted to make Macbeth a somewhat sympathetic figure so that his downfall would seem somewhat tragic. Part of the blame for Macbeth's murder of Duncan is shifted onto Lady Macbeth. She really seems to exist as a character for that sole purpose, and she tends to fade out of the picture after her purpose is served. We feel certain that Macbeth would never have committed that crime if his wife hadn't goaded him into it in every way she could think of. At one point early in the play he tells her that they are going to abort the plan. He tries to be firm when he tells her:
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. Act 1, Scene 7
But his wife knows how to get around him. She questions his manhood and uses what appears to be sexual blackmail. If he doesn't kill the king it will prove he doesn't love her. She is stronger than he is. This manipulation is intended to make him seem less guilty of the murder, but at the same time it makes him look weak and uxorious.
The three witches also seem to share in Macbeth's guilt. They deceive him with false prophecies. They make it appear that he has no choice but must murder Duncan because Fate has decreed it.
Then there is the matter of Macbeth's conscience. This is intended to try to preserve some degree of audience sympathy for the tragic hero even after he had done a despicable deed. He doesn't want to kill the king, and he feels terrible remorse after he has done it. He suffers excruciating psychological pain--fear, guilt, regret, sorrow, shame--when he is forced to be present outside the king's chamber as Macduff goes inside and discovers the dead body all covered with blood. Macbeth tells himself:
Had I but died an hour before this chance,
I had lived a blessed time; for from this instant
There's nothing serious in mortality:
All is but toys; renown and grace is dead;
The wine of life is drawn, and the mere lees
Is left this vault to brag of. Act 2, Scene 3
By "this vault" he means the empty shell of himself. But what's done cannot be undone. Having sold his soul to the devil, "the common enemy of man," Macbeth feels he has nothing more to lose, and he goes from bad to worse. As he tells his wife:
I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er. Act 3, Scene 4
Everybody hates and fears him. Everybody knows he killed Duncan, Banquo, and Macduff's family. The only thing left to admire about this once honorable man is his courage. He has the courage to challenge Fate itself to mortal combat. He goes down fighting.