What are the relations of Act 2:1 and 2 to the rest of the play in Macbeth?

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.