Lady Macbeth realises that Macbeth is not convinced by her persuasions and she tries to entice evil into herself: “Come, you spirits that tend on mortal thoughts, unsex me here and fill me from the crown to the toe upfull of direst cruelty.” She also tries to get rid of her conscience so that the act of killing Duncan will not be painful for her: “Make thick my blood, stop up th’access and passage to remorse that no compunctious visitings of nature shake my fell purpose nor keep peace between th’effect and it.”
Lady Macbeth almost certainly turned Macbeth to commit regicide after he had rejected the idea many a time: “We will proceed no further in this business.” He knows his only motive for killing Duncan is his “vaulting ambition” and the consequences if he is found out are dire. He inherently knows what he might do is wrong and that Duncan is a good person: “He hath honour’d me of late.”
After hearing from her husband that he does not want to kill Duncan, Lady Macbeth flies into a rage and accuses him of being disloyal: “What beast was’t then that made you break this enterprise to me?” She says that she would rather kill her own child than be untrustworthy to her husband: “I would, while it was smiling in my face, have pluck’d my nipple from his boneless gums and dash’d the brains out, had I so sworn as you have done to this.”
She then tries to encourage him and tells of her plan to make it look like the chamberlains killed Duncan. Macbeth is hurt by her criticism and also encouraged by her plan to cover the crime up. At this, he relents and agrees to kill Duncan. He is for the first time excited about the prospect of killing Duncan and tells Lady Macbeth that they have to cover their emotions: “False face must hide what the false heart doth know.”
Although Macbeth was perhaps coerced by Lady Macbeth into killing Duncan, after Duncan’s death and Macbeth’s subsequent rise to King of Scotland, he becomes a more authorative person. He rids himself from the controlling influence of Lady Macbeth and becomes the dominant character in their relationship. He made the decision to kill Banquo without Lady Macbeth’s assistance. He saw Banquo as the one obstacle between himself and a safe reign: “We have scorch’d the snake, not kill’d it.” His insistence on killing everyone who he feels he is threatened acts to highlight his insecurities. Macbeth was not content with his rule and it is perhaps his own self-doubt that leads to his downfall.
The witches’ prophecies told Macbeth that Banquo’s children would be kings of Scotland: “Thou shalt get kings, though thou be none.” Although Macbeth decided alone to kill Banquo, it was perhaps with the witches’ prophecies in his mind that he chose to do such an act.
Eventually, perhaps Macbeth realised his wrongdoings; “Wake Duncan with thy knocking I would thou couldst’” and he sub-consciously wanted to be punished for his wrongs by Macduff. His refusal to fight Macduff but at the same time not give up like a coward was Macbeth’s way of trying to correct himself: “I’ll not fight with thee… Lay on, Macduff, And damn’d be him that first cries, ‘hold, enough!’”
Macbeth’s reignited ambition could perhaps be the cause of his own downfall. He began to think of himself as invincible towards the end of the play and decided to kill Macduff despite the witches saying that it was unnecessary: “Then live, Macduff, what need I fear of thee? But yet I’ll make assurance double sure and take a bond of fate: thou shalt not live.”
Macbeth’s eventual death can be attributed to his own wrongdoings after becoming King. However, one could argue that Macbeth’s death is different to his downfall. One could say that his downfall began the moment he killed Duncan. One might also say that being King was part of Macbeth’s downfall and that his abuse of power was not a cause of his downfall, but a part of it. Whatever occurred after Duncan’s death is maybe irrelevant as his downfall began when he killed Duncan.
Thus, I believe that whoever persuaded Macbeth to kill Duncan is responsible for his downfall. The witches’ prophecies put ideas into Macbeth's head but did not make him kill Duncan. I do not believe that Macbeth wanted to kill Duncan and that it was Lady Macbeth who forced him to, perhaps even against his own will.
Therefore Lady Macbeth is the main reason for Macbeth’s demise, so although Macbeth killed Duncan and started his own decline, Macbeth is responsible for his own downfall to only a small extent.