I believe the strongest argument for what caused the tragedy in Macbeth is offered above. To offer another idea, we might look at the role responsibility plays in the death of Duncan.
Macbeth doesn't come up with the idea for murder by himself but is persuaded by both his wife and the witches. For this reason he is able to say yes to the murder without, initially, being responsible for the act.
Later he finds that he is the only person who will be held responsible for Duncan's murder (and Banquo's murder), as the witches are not liable really and Lady Macbeth is dead.
The tragedy, seen in this light, comes in the fact that Macbeth let himself be talked into a crime then suffered the consequences of his acquiesence. He was tragically gullible.