(Click the character infographic to download.)
Duncan is the King of Scotland, but he might as well be your dad. We should all be so lucky: he's kind, generous, benevolent, and just a little weepier than you might expect from a noble warrior and king. Even Lady Macbeth, who says she would murder her own nursing babe, can't kill him because, as she says, he "resembled/ My father as he slept" (2.2.16-17). Is this the man who should be king?
Duncan is totally the Early Modern version of a sensitive, New Age guy. Post-battle, when he's chilling with his advisors, he can't contain himself: "O valiant cousin! worthy gentleman!"; "So well thy words become thee as thy wounds;/ They smack of honour both"; "worthy thane"; and "Great happiness!" (1.2.26,47-48;55;67). But—did he fight at all? Or did he just sit around, waiting for everyone else to do the disemboweling?
And then, later, when he's announcing his heir, he can barely choke the words out:
My plenteous joys,
Wanton in fulness, seek to hide themselves
In drops of sorrow. (1.4.39-41)
That's an awfully fancy way of saying, "I'm crying." We're not saying that men (or kings) shouldn't cry; but we are suggesting that, just maybe, you want a little backbone from your lord.
On the other hand, making Duncan into such a great guy emphasizes the enormity of killing him. Even Macbeth realizes it, saying that Duncan "Hath borne his faculties so meek, hath been/ So clear in his great office, that his virtues/ Will plead like angels, trumpet-tongued, against/ The deep damnation of his taking-off" (1.7.17-20). Translation: Duncan has been such a good, mild king that murdering him would be completely awful. That Macbeth can murder this man shows us just how atrocious the act is. It's also a clear indication that Macbeth is far removed from human kindness and morality.
Duncan also lets us think about the play's treatment of masculinity. Remember how Macbeth is always worried about being a man (thanks to his wife constantly insinuating that he isn't much of one)? Well, if Macbeth thinks that being a man is all about waving a pointy stick around, Duncan doesn't seem like much of a man.
But don't take our word for it. Take the word of Shakespeare scholar Janet Adelman:
Heavily idealized, this ideally protective father is nonetheless largely ineffectual: even when he is alive, he is unable to hold his kingdom together, reliant on a series of bloody men to suppress an increasingly successful series of rebellions…For Duncan's androgyny is the object of enormous ambivalence: idealized for his nurturing paternity, he is nonetheless killed for his womanish softness, his childish trust, his inability to read men's minds in their faces, his reliance on the fighting of sons who can rebel against him. (source)
Translation: Duncan might be a good father, but he's not a very good king. He needs other men to fight his battles, and he can't even tell when those men are about to betray him. Riding up to the Macbeth's castle, he thinks it looks like Club Med: "This castle hath a pleasant seat; the air/ Nimbly and sweetly recommends itself/ Unto our gentle senses" (1.6.1-3). Shakespeare may not be saying that Duncan deserved to die, exactly, but does seem to be saying that we shouldn't be surprised when he does.
King Duncan is a lot like the historical figure Duncane from Shakespeare's main source for the play, Volume II of Holinshed's Chronicles of England, Scotland, and Ireland. In the Chronicles, Duncane is too "soft and gentle of nature" and is contrasted with Macbeth, who is "cruel of nature." Shakespeare picks up on this contrast in Macbeth. If, on the one hand, King Duncan is too gentle and Macbeth, on the other hand, is a tyrant when he becomes king, then is the play calling for something in between —a king that rules with authority and mildness?
And is Shakespeare slyly suggesting that King James I, who traced his lineage back to Banquo, just might be that guy?