Who is the antagonist in Macbeth by William Shakespeare?

This depends on your point of view. Some people blame Lady Macbeth for manipulating her husband. Some people blame the witches. I'm inclined to consider Macbeth's own nature to be the major antagonist.

You need to consider that nobody made Macbeth do anything. Macbeth's tragic flaw, all protagonists in all tragedies have a tragic flaw, is his greed and ambition combined with the fact that he is kind of a coward. He stumbles his way through successive murders until he becomes King. Unfortunately by that time, he has pretty much sold his soul and become a socio-path that nobody likes.