Two of the things that render Macbeth a tragic hero is his unfailing loyalty to his cause and his complete belief in the prophesies of the witches to the point that they control his actions. He believes he has a "charmed" life and, just as his valor in battle ensured that Duncan lavished rewards on him - making him Thane of Cawdor- so will his misplaced bravery supposedly save him this time.
Macbeth cannot perceive of anyone who was "NOT of woman born" and so, despite the advancing wood - as per the witches prophesy- he is still confident that he is invincible.
The final betrayal is when Macbeth learns that Macduff was "untimely ripped" from his mother's womb and Macbeth realises that he is all alone in his quest; no longer having the witches "false" prophesies to rely on. The witches are no more than "juggling fiends" who cannot be believed who persuade and
..keep the word of promise to our ear
And break it to our hope! V.viii.21-22