My answer would be as follows:
First and foremost Duncan symbolizes a true Christ-like figure since he is described as pure and pious. But a "good" tragedy should follow the principles of Aristotle's Poetics i.e. it must trigger fear and pity in the mind of the reader. The fear and pity exerted upon the reader's mind are intended to correct the inner imbalance of the reader. (thanks to the pivotal principle of catharsis)
Of course we can assert that Duncan is characterized by a strong gullibility. Nonetheless, and beyond this perfunctory aspect, we might notice that he is quiet. His quietness is pivotal in this context if we want to understand the true kernel of the play. His quietness is many-facetted since he is reposed physically and mentally. Thus his soul is not gnawed with despair or remorse. Furthermore Duncan is the only character who has a good conscience.That's why he is killed. He is killed because of his purity.
My answer is incomplete but I hope it will be useful.