The start of Act III, Scene 1. Just before Banquo's murder.
Banquo: You have it now, Thane of Cawdor, just as the witches promised. You killed the king of Scotland so you could take the throne, (point to crowd) I know you did it. I fought against the dreams of those predictions and proved myself worthy of what I was told.(point to myself) I defended my character and spoke up for what I stood for. I would not weaken in the face of temptation. Since the day where those foul hags told Macbeth and I our fates, Macbeth has not been himself. As if overcome by some evil spirit, he has changed his ways, actively seeking his future through deceit, what I now no longer suspect him of, but believe he dedicated.
The Thane of Cawdor, he was named shortly after the predictions, he did nothing to receive the title but be his own character, a strong, passionate and loyal countrymen. The real Macbeth we all knew and respected. But the third prediction was the one that changed Macbeth, a title that toyed with his ambition.
Being told he would become King I believe was the beginning of the end for him. Over time his aspirations led him to take a path the true Macbeth would have walked right past (point to the door). He was tempted by the unnatural to go down this path of turmoil and disruption. He lost his way and was not strong enough to fight it off. He began to spend more time by himself; he acted suspicious and began scheming with that haggard wife of his. He was given a chance to take the place of King and he took it, fiercely, unlawfully and shamefully.
Had Macbeth killed the King of Scotland for his own gain? I mean, why else would he kill Duncan for? Macbeth was Duncan’s favourite thane. He was