I think Lady Macbeth played a huge role in the murder of King Duncan. She was the one who persuaded her husband it was a good idea after he had told himself that it wasn't and decided to leave it to chance because after all, the other predictions that the witches had told him had come true.
The witches had planned where they were going to meet Macbeth.
'Upon the heath. There to meet with Macbeth.'
The witches knew that Macbeth was making his way towards them because they were summoning him. They had planned to play with his ambition to make it stronger and find out how far he would go to get what he wanted.
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