By "infirm" Lady Macbeth uses the antiquated understanding of the word: not firm, or unstable. In a previous scene, Lady Macbeth told Macbeth that they could not fail if he shored up his courage. However, with this line, Lady Macbeth recognizes that he was not as firm as she wanted him to be, and thus they are in danger of being caught.

By "sorry sight" Macbeth means two senses of the word: both something painful and distressing and something that induces feelings of remorse or sorrow. Macbeth's comment on his bloody hands reveals his immediately guilty conscious over King Duncan's murder. Lady Macbeth's unsympathetic response that he is foolish also reveals a lot about their relationship. She lacks empathy and respect for Macbeth's feelings or mental state.

This line is significant in that Macbeth wishes the knocking could wake up Duncan. This is the first mention of actual remorse that he has expressed in this scene.

The contrast between how Lady Macbeth and her husband react to the blood is significant. While Macbeth sees it as a symbol of his crime that will not go away, Lady Macbeth considers it evidence that can be removed and appears to have no remorse for playing her role in Duncan's murder. However, notice how Lady Macbeth's perception and attitude toward the crime changes later in the story

The verb incarnadine here means "to redden." Macbeth is saying that the blood on his hands will turn the whole ocean red rather than be cleaned off. Prior to Macbeth this word's meaning was primarily associated with coloring objects red; however, since Shakespeare, the word is most often associated with blood.

In Roman mythology, Neptune is the god of the sea. Macbeth invokes this name to emphasize his belief that all the water in the world will not be enough to wash the blood (guilt) from his hands.

Shakespeare wants both husband and wife smeared with blood. Naturally Lady Macbeth will return with bloody hands and some blood on her garment after smearing the faces of two men with Duncan's blood. In doing this, Shakespeare creates the illusion that the audience has witnessed a horrible murder without showing it on stage.

In an effort to get her husband to stop raving, Lady Macbeth tries to remind him of the necessity for action to avoid getting caught. She tells him to wash the bloody evidence ("filthy witness") of his crime from his hands. As far as she is concerned, the blood is only physical and washing it away will destroy the evidence of Macbeth's crime. Notice how she continually tries to calm Macbeth and remain the voice of reason.

Notice how before murdering Duncan, Macbeth imagined seeing a floating dagger before him. Now, after committing the crime, he has imagined hearing voices. These illusions will continue to plague him as he struggles with his guilty conscience.

These words are filled with dreadful irony and foreshadowing. Considering how Macbeth has already imagined a floating dagger prior to killing Duncan and his current agitated state, the likelihood that one or both of them will mentally suffer as a consequence of this action is a distinct possibility.

Notice the shortness of Lady Macbeth's replies. She is attempting to quiet her husband, and here she calmly states that Donalbain and an attendant are sleeping in the second chamber.

Macbeth's hands are bloody from killing Duncan. While many consider hangmen as only responsible for conducting hangings, at the time they were also tasked with the bloody work of disemboweling and quartering the bodies of the executed—particularly for the bodies of those who committed treason.

This exclamation is an example of Macbeth's overwrought nerves: He imagines he hears a sound while speaking with his wife. The continuing dialogue shows that he imagines that he has heard a voice as well.

Duncan has two sons, Malcolm and Donalbain. This is the first mention of Donalbain in the story. If Macbeth were to ensure his claim to the throne, he would have needed to kill both of Duncan's sons as well.

Shakespeare often made his characters, even villains, complex and sympathetic. Lady Macbeth's reference to her father is an example of how Shakespeare "humanizes" her, and it is also one of the few moments when she reveals herself as a very human character and not simply wicked and hungry for power.

Since Duncan bestowed many gifts on Macbeth's company, there was much drinking and feasting earlier in the evening. This word, surfeited, tells us that Duncan's grooms, or bodyguards, overindulged in food and drink.

A posset is a hot drink of sweetened and spiced milk curdled with ale or wine. Lady Macbeth created a sleeping potion to drug Duncan's guards, and here she indicates that it was so strong that it may have half poisoned them.

As Macbeth leaves the courtyard to carry out the deed, Lady Macbeth enters it. Since there is no change of scene, her brief soliloquy is meant to pass the time until Macbeth returns from Duncan's chamber.

In Renaissance England, the hoot of an owl flying over a home was considered a bad omen, and it meant death for someone inside the house. Shakespeare compares the owl to a bellman, whose job was to ring the church bell when someone in town was near death. This signal let others know to pray for the dying person. In this case, the owl's shriek represents a fatal ringing of the bell for King Duncan.

Since this is Macbeth's tragedy, Shakespeare tries to retain some modicum of sympathy for his hero. The audience is, of course, repelled by Macbeth's cold-blooded murder of Duncan, to whom he is obligated in so many ways. But Shakespeare tries to mitigate the crime by making Macbeth reluctant to go through with it and then making him feel guilty and remorseful throughout the rest of the play. Here, he says he cannot go back to Duncan's chamber because he cannot bear to see what he has done. He cannot even bear to think about it. So his wife takes over. This helps to pass some of the guilt over to her, thereby hopefully diluting some of Macbeth's own guilt. Macbeth really should murder the King's two sons on the same night he kills their father. He will never have a better chance, and they stand in his way to the throne. Duncan has already proclaimed Malcolm his successor. But Shakespeare does not want to show Macbeth murdering two young boys in their beds. That would lose all audience sympathy for the tragic hero. As it turned out, Macbeth was unable to kill the boys on the night he killed their father—if that was his plan—because he lost his nerve; he thought he heard a loud voice crying "Sleep no more!" to wake up the entire house, and then there was the persistent knocking at the gate.

The actor playing Macbeth probably was instructed to pronounce the word "Amen" with difficulty, as in a hoarse whisper, to illustrate the trouble he had trying to say the word in the King's chamber. Shakespeare frequently uses characters' dialogue to inform actors' performances in this way.

This passage of dialogue is intended to "humanize" and also "feminize" Lady Macbeth. She has been acting supremely confident, but once she is alone she is anxious. This is the first time she says, "I am afraid"—afraid of anything. She admits it to herself. She is afraid something went wrong. She realizes their plan was hardly foolproof. A dozen unexpected things could have happened. She would like to be reassured and comforted. When she says, "My husband!" it signifies that she welcomes his strength; he is someone to lean on.

This whole speech by Lady Macbeth is intended to "humanize" her. It shows that she is not nearly as confident as she has been acting with her husband. She is good about hiding her fears, worries, and guilt from other people, but she is human. Shakespeare usually tried to show that all his characters were mixtures of good and bad, strength and weakness. That was why he showed the wicked King Claudius at prayer in Hamlet. When Shakespeare has Lady Macbeth say

Had he not resembled
My father as he slept, I had done't

the purpose is consistent with everything she says in this speech. It shows that she has a tender side to her nature. She is not only a real human being but a real woman. She loved her father and is not a monster.

Because of her social status and responsibilities as a woman, Lady Macbeth is actually dependent upon her husband to fulfill her ambitions. Despite these ambitions, however, the fact that she says she would have killed Duncan if he had not resembled her father does not necessarily prove she really would have done it. Shakespeare often has characters rationalize to explain their behavior to themselves. Hamlet is full of such rationalizations, and the titular character of King Lear threatens to do terrible things to his daughters although he must realize, at some level, that he has become totally powerless. Lady Macbeth expresses a lot of vicious intentions, but she leaves the dirty work up to her husband.

It seems likely that Macbeth might have been planning to kill Malcolm and Donalbain that same night. Shakespeare gives several reasons why Macbeth would have had to abandon that plan and retreat to his own chamber. One is that he is afraid one of the grooms might have seen him standing there "with these hangman's hands." Another is that he thought he heard a strange voice crying, "'Sleep no more! to all the house.'" A third reason is that he just lost his nerve. He won't even take the grooms' daggers back to Duncan's bedchamber. Then a loud and prolonged knocking at the gate makes it out of the question to do anything but wash up and pretend to be asleep.

This is probably intended to explain why Macbeth returns to his chamber with two daggers. His wife wanted both grooms' daggers to be used on Duncan so that there would be no chance of the murder being construed as only one man's guilt.

Thomas De Quincey wrote a famous essay titled "On the Knocking at the Gate in Macbeth" in which he focuses on the effectiveness of this offstage sound effect. However, the knocking has a more important purpose. It may force Macbeth to open the gate himself, because the Porter and the entire household staff are drunk, and although he had planned to pretend to be sound asleep, Macbeth will have to be present when Duncan's body is discovered.

If he has any hope of being elected king, Macbeth certainly should plan to kill Duncan’s two sons the same night he murders their father. Shakespeare offers a number of possible reasons why Macbeth does not at least try to do so. Apparently Macbeth doesn’t know where to find the two boys. Furthermore, he will soon describe hearing or imagining a voice crying out “to all the house,” and he is standing there with two daggers and bloody hands. (The fact that Macbeth is still carrying the daggers suggests he intended to use them on Malcolm and Donalbain.) Then there will be a knocking at the gate which keeps getting more insistent. If Macbeth had any thoughts of killing the king’s sons, he not only has to abandon them but has to wash up quickly and go down in his nightgown to open the gate.

Shakespeare has Lady Macbeth go to Duncan's chamber because he wants her to get covered with blood too. That is the main reason Shakespeare has Macbeth return with the two daggers. These daggers also, of course, symbolize the murder performed offstage. 

As much as Lady Macbeth covets seeing her husband as king, Duncan's virtue and his father-like goodness makes it difficult to justify his murder.

Here, Macbeth admits that his murder of Duncan speaks against his true character. He no longer likes who he is since murdering Duncan. 

Macbeth realizes that nothing will clean (wash) the blood from his hands. Now that he has murdered, his hands will always be covered with blood (figuratively), and he will never be free of guilt. This is also an allusion to Pontius Pilate and his washing of his hands (stating that he was innocent of Jesus' blood which was spilled during his crucifixion). 

Given the supernatural elements present in this play, it is impossible to say for certain whether Macbeth hears or imagines this voice. However, its words accurately foreshadow his fate. The insomnia Macbeth suffers due to his guilt will contribute to a blurring of the imaginary and the supernatural, as he continues to perceive sounds and visions that may or may not be real.

This quote illustrates the fact that Macbeth recognizes his sin (since he tries to pray). Unfortunately, "Amen" gets figuratively stuck in his throat, indicating that one who just sinned should not be able to immediately ask forgiveness. 

Without the ability to conjure complex sound effects for his stage, Shakespeare often uses dialogue to describe to the audience what the characters are hearing. Here, the “scream” of the owl and the “cries” of the crickets describe nature speaking out against Duncan’s murder. His murder and the removal of his kingship upset the natural order.