Macbeth was responsible for his own downfall because Banquo ignored the witches, but he latched onto their predictions as absolutely having to come true. He also wrote to his wife about them.
Macbeth is fully aware that he should not be king. He has no claim on being king. The most logical successor is Duncan’s son Malcolm. In an aside, Macbeth comments on his ambition and how he hopes no one else notices it.
My worthy Cawdor!
[Aside] The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires … (Act 1, Scene 4)
Evidence of Macbeth’s role in his own downfall is his reaction to Lady Macbeth. First of all, he did tell his wife about the witches. That was not necessary. He could have kept it to herself. Although his wife nagged him about killing the king, the choice was his own.
Is this a dagger which I see before me,
The handle toward my hand? Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight? or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain? (Act 2, Scene 1)
Macbeth looked at it from all sides, after all. He realized that he should not kill the king because he was his kinsman and his host, and because Duncan didn’t deserve it. He was the one who made the choice to kill the king.
However, you could say that Macbeth’s downfall came mostly from the fact that he continued killing after Duncan, in order to maintain his role as king. He had Banquo and the Macduff family killed (except Macduff himself). That was not Lady Macbeth’s fault. She didn’t know anything about it.