At the start of the play, Macbeth is every bit the hero. Noble, brave, and unswervingly loyal to his king, Macbeth is renowned throughout the kingdom as the greatest warrior in the whole of Scotland. More than anyone else, he was responsible for the crushing defeat that the Scottish forces have just inflicted upon the Norwegian enemy. And King Duncan is so grateful for Macbeth's heroic exploits that he awards him with the title of Thane of Cawdor. A just reward, one might think, for a display of such remarkable courage.
And yet there's an ominous element of foreshadowing here. The previous Thane of Cawdor betrayed Duncan by going over to the other side. And in due course, Macbeth will betray Duncan, despite everything he's ever done for him, by brutally murdering him. That an ostensibly loyal and noble warrior should stoop to such base treachery is truly shocking indeed.
The contrast between the Macbeth that we see at the start of the play and the bloody, murdering tyrant that he becomes after killing Duncan really could not be greater. That Macbeth undergoes such a profound transformation is what makes him such a complex, fascinating character. On the one hand, he acts like a bloodthirsty tyrant, yet at the same time, he is never entirely comfortable in his new role as king. This would suggest that Macbeth's transformation, though undoubtedly remarkable, is nonetheless far from complete.