Macbeth now knows he does not need to worry about other men but still is going to kill Macduff so he can guarrantee his survival.
I do not need to kill Macduff but I will anyway to assure my survival.
Another spirit comes to tell the last prophocie of Macbeth. He is told that he will never be defeated until Birnam Wood shall fight him on the Dunsinane Hill.
No one shall defeat Macbeth until Birnam wood faces him on the Dunsinane Hill.
Wait what?
Macbeth has now left the witches and asks his servant if he had seen the witches as he passed by.