Lady Macbeth, who so encouraged Macbeth to murder Duncan so that the witches' prophesies can be realised in Macbeth, is still fearful as to whether Macbeth will be discovered after having committed the deed that will make him "more the man" (I.vii.51)
After some light banter between the porter, who admits to some drinking and carousing, and Macduff, and a discussion on the weather where we understand that there was a storm the night before which made "the earth feverous" (II.iii.58), the suspense builds regarding the imminent discovery of Duncan's murder. The "strange screams of death"(54) heard during the storm increase the tension as the audience knows that this can only be the witches.
MacDuff rushes to tell Macbeth and Lennox who are waiting whilst he wakes the king, that "sacrilegious murder" has taken place and "stole..the life of th' building." (65-6) Macbeth plays along, shocked at such news and he and Lennox leave. The bell rings to alert everyone.
Interestingly, it was a bell that prompted Macbeth to finally commit to his misdeed, as if in a trance, and a bell now rings as the deed is clearly accomplished.
Lady Macbeth also plays her role well, expressing her shock. She is aware that Macbeth took it one step further by murdering the guards but now it is almost too much for her and she faints. The others will presume that it is her sensitivities that made her faint and not her fear at the enormity of what Macbeth has done. Such begins her descent into madness.
Macbeth , in the meantime, is becoming resourceful and even brazen and admits to killing the guards to abate his "fury" (104) as they must have killed Duncan. Donalbain and Malcolm, under Banquo's advice, recognize that their may be "treasonous malice" (131) and are cautious to trust "the near in blood" (139) who may have motive and are "the nearer bloody." (140)
The scene is now set as "there's no mercy left."(145)