How does Macbeth's brutal death in Act V, Scene 8 parallel another event early in the play?
Macbeth orders the killing of Banquo and Fleance. Fleance escapes. And Macbeth orders the killing of Macduff's family. So, we don't really see the violence of these acts and Macbeth himself is not present when these murders occur. However, Macbeth personally kills Duncan and returns with the blood on his hands to prove it. So, if there is a reciprocating parallel (or a violent kind of poetic justice), it is between Macduff killing Macbeth and earlier at the beginning of Act 2 when Macbeth kills Duncan.
Near the end of the play, Macbeth tells Macduff that he is protected by the witches' prophecy that he cannot be killed by "one of woman born." So, Macbeth thinks he is invulnerable. However, Macduff was born by way of a Caesarean section, meaning he was removed from his mother's abdomen, a technicality which does not apply to the witches' prophecy. The ironic parallel is that Macbeth killed Duncan when he was most vulnerable: in his sleep. Macduff kills Macbeth when Macbeth thinks he is invulnerable. Of course, he is not invulnerable and Macduff kills him.
Another interesting parallel between Macbeth's brutal death and one he has inflicted is described in the very beginning: Act I, Scene 2. This is the Captain's reports of how "brave Macbeth" (how ironic this epithet is at the end!) brandished his sword and slay Macdonwald as he "unseamed him from the nave to th' chops." Thus, Macbeth's method of causing Macdonwald's death has an inverted parallel as Macbeth is defeated by a man born of a woman who was "unseamed" in the birthing process. This, indeed, is a violent kind of poetic justice as mentioned above. And, with these inverted parallels of Macbeth's initial actions, the motifs of "fair is foul, foul is fair" and "nothing is what it seems" are fulfilled in Act V.