Is King Duncan in Macbeth a discerning man and a good judge of character?
The nature of Duncan's murder and the events leading up to it suggest that he was neither a discerning man nor a good judge of character. Duncan was an honorable and trusting man. When he comes to Macbeth's castle for the night, he believes he is honored and beloved by Macbeth and Lady Macbeth. He takes them at their words, enjoying the warmth and good cheer of their welcome, unaware that they have already planned his murder that very night.
In defense of Duncan, he had no reason to doubt Macbeth's and Lady Macbeth's loyalty, and he had strong reason to trust Macbeth's honor and fealty, especially considering how valiantly Macbeth recently had fought for him against the traitor Macdonwald and the King of Norway. Duncan's own nature was good and true; unfortunately, he attributed these same traits to Macbeth and his wife.
Ironically, after Macdonwald had betrayed him, Duncan himself noted how difficult it is to identify a traitor:
There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust.
Duncan's experience with Macdonwald foreshadows Macbeth's treachery and treason.