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.
Returning from the battle Macbeth and Banquo are confronted by the witches. Macbeth doesn't know that the witches are going to trick him. The reader or audience know something is going to happen because Macbeth echoes something that the witches had said earlier on when they were planning to meet with him.
'So foul and fair a day I have not seen.'
When the witches are about to leave they say,
'Fair is foul, and foul is fair.'
When the witches realised Macbeth and Banquo are there they address Macbeth,
'All hail, Macbeth! Hail to thee, Thane of Glamis! All hail, Macbeth! Hail to thee, Thane of Cawdor! All hail, Macbeth! That shalt be king hereafter.'
Banquo then gets jealous that Macbeth is the only one being told prophecies so he asks what is in store for him.
'Thou shalt get kings, though thou be none.'
The witches manipulate Macbeth's ambition to get greater things. The story is made into a tragedy because Macbeth doesn't even know what the witches are doing and does what they want him to do.