Macbeth--a man who earns respect because he distinguishes himself through his heroic deeds, proves to have a moral conscience, but unfortunately possesses a character flaw--ambition--that destroys him and everyone close to him.
The deep guilt Macbeth feels humanizes him and moves the audience.
Recall that Macbeth's noble character is established from the start. A dark side of Macbeth is revealed, savagery even in the way he kills the opponent Macdonwald: "Till he unseamed him from the nave to th' chops, And fixed his head upon our battlements" (Macbeth 1.1).
As soon as Macbeth receives the new title as Thane of Cawdor from king Duncan, his ambition is inflamed. However, he still knows the line between right and wrong and is firmly standing on the side of good.
At the very first thought of removing king Duncan from the throne so that he could be king, Macbeth feels terrified and ashamed. "Stars, hide your fires, let not light see my black and deep desires" (Macbeth 1.4) He is ashamed to bring to light this dark side of his psyche, this greedy ambition that would lead him to have such evil thoughts.
After Lady Duncan strongly urges Macbeth to go forward with the murder plot, Macbeth still carefully reasons the pros and cons of the plan. He thinks about his responsibility, his loyalty, about morals. He announces firmly to his wife that they will not proceed any further in this business (1.7 paraphrase). He knows it is wrong, period.
Sadly, he succumbs to her manipulation and an attack on his manhood, and agrees to her murder plot.
The play is known for its blood imagery that highlights both Macbeth's, and later, Lady Macbeth's guilt. As soon as he commits the murder, Macbeth begins his downward spiral of self destruction from unbearable guilt. The famous lines that show his deep remorse for killing Duncan are when he looks at his murderer's hand in disbelief ("What are these hands?" he asks, "They pluck out mine eyes!"). He says that he has murdered sleep and will never again have the peaceful rest again.
Examing the magnitude of Macbeth's guilt in these lines:
"Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas incarnadine,
Making the green one red." (2.2)
Macbeth's tragedy is magnified by his own recognition of his mistake. In his famous soliloquiy at the end of the play he looks at his life's accomplishment and what he can be proud of. Nothing. Re-read carefully his speech "Out, out, brief candle." He says that life is like a tale told by an idiot, full of sound and fury, signifying nothing. He recognizes that his climb to the top has only brought him misery, pain, and absolute destruction. He has lost his humanity, ability to feel, and his conscience. He has become a monster, and he feels ashamed.
Lady Macbeth is a whole separate psychological study of guilt because for a long time she completely denies it, but she, too, is destroyed by it as she sleepwalks under the great burden of a guilty conscience, loses her mind, and eventually takes her own life.