It is fitting that the play ends as it began: with a victorious battle in which a valiant hero kills a traitor and displays his severed head. The first thing we hear of Macbeth in act one is the story of his bravery in battle, wherein he cut off MacDonald's head and displayed it on the castle battlements. Here at the end of the tragedy, Macbeth, himself a traitor to Duncan and his family, is treated in exactly the same manner; after killing Macbeth, Macduff enters with Macbeth's severed head and exclaims "behold where stands / Th'usurper's cursed head". The play thus ends with the completion of a perfect parallel.
The moral at the end of the story is that the course of fate cannot be changed. The events that the Weird Sisters predicted at the beginning of the play happen exactly as they said, no matter what the characters do to change them. Macbeth tries his hardest to force fate to work to his bidding, but he is not successful; Banquo still becomes the father of kings, and Macbeth still falls to a man not born of woman. The man who triumphs in the end is the one who did nothing to change the fate prescribed for him.
In-depth summary of important points in the scene
As the play nears its bloody conclusion, Macbeth's "tragic flaw" comes to the forefront: like