In Shakespeare's Macbeth, Act 2:1 and 2 are the culmination of what comes before, and the catalyst for what's to follow.
The witches' predictions, Macbeth's and Lady Macbeth's interest in the predictions, the idea that what appears fair is really foul, and ambition all lead to the assassination of Duncan in scenes one and two of act two.
And from that assassination come the deaths of Banquo, Macduff's family, Lady Macbeth, and Macbeth himself. From that assassination come Lady Macbeth's guilt, Malcolm uniting with Macduff, and Macduff's killing of Macbeth.
The assassination of Duncan is the defining act of the play. The themes of the play are introduced before the assassination (ambition, the unnatural, fair is foul, equivocation, gender roles, etc.), and expanded on and played out after. The killing of a king and kinsmen is an act from which there is no going back. From that point on, Macbeth is forced to safeguard his position, and forced, in his mind, to eliminate all threats to his reign.
There are also some details in each scene that have important ramifications later in the play. In Act ii, scene i, Shakespeare hints at the 180 degree turnaround that Macbeth has done in just three short scenes since Macbeth and Banquo have been onstage together.
First, when he enters and Banquo (unable to see who it is in the dark) asks, "Who's there?" Macbeth replies, "A friend." We cannot know for certain if the means this sincerely at this point. Already, Macbeth is becoming a character that may or may not speak the truth. We also don't really believe Macbeth's assertion that he "think[s] not" of the weird sisters. The friendship that seemed so hearty in the opening of the play seems strained and guarded. This foreshadows Macbeth's betrayal and murder of Banquo in Act III.
In scene ii, there is some dramatic irony set up here, as Macbeth seems the jumpy and spooked one by noises and sounds and the blood and gore. Lady Macbeth is fearless and seems without any concern for the dastardliness of the deed. However, by Act V, she will have become completely consumed with sleepwalking vision of the blood and guilt associated with this murder, while Macbeth has turned into the cold-blooded killling machine.