What is Duncan's debt to Macbeth?

In Act 1, Scene 4, King Duncan thanks Macbeth profusely for championing him against the invaders. Duncan ends by saying:
Only I have left to say,
More is thy due than more than all can pay.
This means Duncan can never sufficiently repay Macbeth; even if Duncan gave Macbeth his whole kingdom, it still would be insufficient recompense. This may be another way in which Shakespeare is trying to keep Macbeth from seeming like a complete villain when he seizes power. After all, Macbeth could twist Duncan's statement of gratitude to mean that he is entitled to anything from Duncan, even the crown.
Shakespeare wanted his play to be a tragedy and for Macbeth to be a tragic hero. He had to preserve some modicum of audience sympathy for his protagonist.
The playwright tries to pin part of the blame for Duncan's assassination on the Three Witches. If they hadn't deceived and encouraged him, Macbeth wouldn't have gone ahead with it. They make it seem as if he can't help himself; it is already predetermined that he will become king--and the way to do that is obvious. Then Shakespeare makes Lady Macbeth take at least half of the guilt. Her husband really doesn't want to kill Duncan and tells her so. He wouldn't have gone through with it if she hadn't forced him.
Even Banquo helps to make Macbeth look less bad. The honorable Banquo is perhaps also tempted by the witches prophecies. In Act II, Scene 1 he says:
Merciful powers,
Restrain in me the cursed thoughts that nature
Gives way to in repose.
The fact that Duncan and his two sons are staying overnight in Macbeth's castle makes it seem as if fate is conspiring with Macbeth by offering him the opportunity he needs to do what he and his wife have been discussing for some time. As she tells her husband:
Nor time nor place
Did then adhere, and yet you would make both:
They have made themselves, and that their fitness now
Does unmake you. (I.7)
In the end Macbeth has become such a dastardly tyrant that the only thing heroic about him is his courage in defying Fate itself.