In the first act of "Macbeth," King Duncan of Scotland describes Macbeth as "noble" (I,ii,66). That he proves to be a poor of judge of character is one of the situational ironies of the play. For, of course, he is slain by this "noble" Macbeth who aligns himself with witches in his evil desire for power.
Earlier in Act I some of Macbeth's savage tendencies are even foreshadowed as the captain describes how Macbeth fought the Irish foot soldiers and heavily armed soldiers (the "kerns and gallowglasses"):
Disdaining fortune, with his brandished steel,/Which smoked with bloody execution,/Like valor's minion carved out his passage/Till he faced the slave;/Which nev'r shook hands, nor bade farewell to him,/Till he unseamed him from the nave to th'chops,/And fixed his head upon his battlements (I,ii,17-23)
It is evidence of his poor judgment of character that Duncan responds to this knowlege with "O valiant cousin! Worthy gentleman!" (I,ii,24), for it seems that Macbeth has brutally slaughtered a man, decapitated him and impaled the head for all to see. After this, the king bestows the title of Thane of Cawdor upon Macbeth, who has killed this "traitor."
The dramatic irony of Duncan's line "What he hath lost, noble Macbeth hath won" cannot be lost upon the audience as it is the traitorous Macbeth who later slays King Duncan who with loving comments ("we love him highly" (I,vii,29) has just been a guest at Macbeth's castle.
In his short span of life within the play, Duncan describes Macbeth as "worthy" in many ways, heaping praise on the new Thane of Cawdor. Interestingly enough, Duncan admits that he mistakenly trusted the last Thane of Cawdor. The same is true when Macbeth is given the title. When first told of Macbeth's newest and most valiant victory, Duncan proclaims, "O valiant cousin! Worthy gentleman!" (1.2.24). It isn't long after that Duncan gives Macbeth his new title of Thane of Cawdor (fulfilling part of the witches' prophesy). A few scenes later, Duncan describes Macbeth as "My worthy Cawdor!" even as Macbeth grumbles at how Malcolm has gotten closer to the throne (1.4.48). Before long, the king is a guest of Macbeth and Lady Macbeth. The hosts, of course, beat King Duncan home, so Duncan has this to say:
We coursed him at the heels, and had a purpose / To be his purveyor: but he rides well, / And his great love, sharp as his spur, hath holp him / To his home before us. (1.6.21-24)
In other words, Duncan describes Macbeth as a fast rider and a skilled one as well. More interestingly, though, Duncan describes Macbeth's wife as "sharp as his spur" who was a great help to him. Ha! You got that right, Duncan. Lady Macbeth is more than just a help. She is the is the chief instrument of Macbeth's ambition! Without her, one must ask whether Macbeth would have killed Duncan in the first place! The final words Duncan has for Macbeth are an interesting description as well:
Conduct me to mine host: we love him highly, / and shall continue our graces towards him. (1.6.29-30)
Duncan uses the word "love" to describe Macbeth here! Duncan is, therefore, increasing his great affection for Macbeth (as the opposite is true for the latter). Duncan also vows to continue lavishing his kingly grace upon Macbeth. Unfortunately, Duncan is killed by Macbeth's own hand before Duncan can do so.