What does Duncan call Macbeth when he hears Macbeth has defeated Macdonwald?
Duncan calls Macbeth valiant and worthy when he hears about his defeat of Macdonwald.
Although we know that Macbeth becomes a murderer and a tyrant, at the beginning of the play he is a loyal and brave soldier. The bloody sergeant tells Duncan about how Macbeth defeated the traitor Macdonwald in battle. He calls Macbeth brave.
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage
Till he faced the slave … (Act 1, Scene 2)
Duncan, the king, hears that Macbeth mowed through a bunch of soldiers to get to the traitor and then killed him, cutting him in half. The king is very impressed by these actions, which are indeed the act of a brave soldier.
O valiant cousin! worthy gentleman! (Act 1, Scene 2)
It is because of these actions that Duncan awards Macbeth the promotion to Thane of Cawdor. Under normal circumstances, I would think that that would be enough. However, Macbeth has been told by three witches that he will be king. He is unhappy to learn that he is still a thane. His ambition has been aroused.
Duncan clearly has no idea who he is dealing with here. He thinks of Macbeth as his kinsman and friend. He goes to stay at his house! He feels that Macbeth will be thrilled with this promotion, and thinks that no one would question his naming Malcolm his successor. Malcolm is his son. It makes much more sense, to everyone but Macbeth.