In the play Macbeth, the main character gets what he deserves. There is poetic justice served in the end of Macbeth. Macduff beheads Macbeth. Justice is served. After all, Macbeth killed Macduff's family. Macduff gets his revenge at the end of the play.
I believe modern audiences still like to see poetic justice served at the end of movies or plays. Everyone loves to see justice served. It is only fair that the evil character receives punishment for his sins.
Macbeth became an evil character. He kills King Duncan and his guards. He shows no mercy to King Duncan who was a guest in Macbeth's house. Then, Macbeth has Banquo killed. He proceeds to have Macduff's wife and children murdered. No doubt, revenge is sweet when Macduff beheads Macbeth at the end of the play. The audience appreciates when the good guy finishes in a victorious manner. Macduff is the good guy and he ultimately pays Macbeth back for all the evil that Macbeth committed.