At the beginning of the play, Macbeth is hailed as one of the best, most loyal and most courageous warriors by everyone. He helps king Duncan defeat the Norwegians, and, as a result, he becomes promoted to the Thane of Cawdor:
No more that thane of Cawdor shall deceive
Our bosom interest: go pronounce his present death,
And with his former title greet Macbeth.
In this speech, King Duncan states that he will have the former Thane of Cawdor executed for his treachery and that his title will be given to Macbeth.
Although Macbeth is promoted by the king, we begin to notice that he has other ambitions. The witches' prophecy encourages him to go after his ambitions, and it becomes evident that Macbeth is prepared to cast aside both his good reputation and his loyalty to the king in order to achieve his personal goals.
Therefore, Macbeth's downfall starts when he no longer thinks his courage and fighting skills are important. The moment he begins working on his plan which pertains to murdering his benevolent king Duncan is the moment he rejects the true values he should live by. His desire to take the throne will only cause the beginning of his downfall.