How is Macbeth presented in a positive way at the beginning of the play?

Macbeth is portrayed as a valiant, honorable Scottish noble at the beginning of the play. The first description we get of the play's title character is given by a sergeant to King Duncan. The sergeant describes a rebellion by Macdonwald, a rebellious nobleman who apparently seeks to overthrow Duncan. Macbeth, a Scottish thane (nobleman), fights his way through the rebel army before slashing Macdonwald "from the nave to the chaps" with his sword and decapitating him. Of course modern readers might be shocked by the violence of this act (which, it might be argued, foreshadows some of the violent deeds Macbeth commits later, in addition to his own demise at the hands of Macduff) but Duncan is impressed by the bravery and loyalty of his kinsman. He calls him a "valiant cousin" and "noble kinsman." Macbeth then leads his forces against an attempted invasion by a "Norweyan lord," again emerging victorious. So Macbeth is respected, even viewed as a hero, by the King and by his peers. This positive portrayal of the man early in the play makes his descent into murder and treachery all the more dramatic.