Perhaps one of the best scenes in which Macbeth's shift in character from noble to dishonorable would be in Act 3, Scene 1 when Macbeth plans and discusses the upcoming murder of Banquo.
Before this point in the play, it has been established that Macbeth and Banquo are friends. However, due to the prophesies of the witches in which they said Macbeth would be king but it was Banquo's line that would last on the throne, Macbeth has become jealous and suspicious of Banquo.
Since Macbeth killed to attain the throne he believes Banquo would not hesitate to do the same. In the murder of Duncan, Macbeth needed much encouragement from his wife to complete the task, but in his meeting with Banquo's murderers, Macbeth is the one with the plan. He does not hesitate nor does he involve his wife in the conspiracy, showing that he is distancing himself from her and thus, is no longer in need of her provoking to do horrible things.
Macbeth's assassination plan is cunning and clever, showing the reader how he has developed into a cold, calculating murderer who would see his best friend killed before losing a crown so wrongfully earned.