What is the contrast between Macbeth at the beginning of the play and later in the play?

At the beginning of the play, Macbeth is portrayed as a honest, valiant warrior, loyal to his kinsman King Duncan. He plays a leading role in crushing the rebellion against Duncan's rule by killing the rebel Macdonwald and, alongside his friend Banquo, marshaling the King's forces to victory. Indeed, the Sergeant describes Macbeth to Duncan as "valor's minion," and "brave Macbeth," describing in gory detail how he killed Macdonwald (and many others) in the battle....

At the beginning of the play, Macbeth is portrayed as a honest, valiant warrior, loyal to his kinsman King Duncan. He plays a leading role in crushing the rebellion against Duncan's rule by killing the rebel Macdonwald and, alongside his friend Banquo, marshaling the King's forces to victory. Indeed, the Sergeant describes Macbeth to Duncan as "valor's minion," and "brave Macbeth," describing in gory detail how he killed Macdonwald (and many others) in the battle. Duncan exclaims that Macbeth is a "valiant cousin" and a "worthy gentleman." 

Yet almost immediately upon encountering the witches and their prophecy, Macbeth's ambition begins to bubble to the surface, especially when he finds out part of their vision has come true after the death of the thane of Cawdor. Still, he is not committed to murdering Duncan to make the prophecy come true, and it takes his wife's repeated goading to urge him onto the act, which he undertakes with deep misgivings, as he reveals in his soliloquy in the final scene of Act I:

I have no spur
To prick the sides of my intent, but only
Vaulting ambition...

Duncan's murder propels Macbeth to the throne, and we quickly witness the corrupting influence of power, especially power gained through violence and treachery. He has Banquo murdered, and though he is clearly haunted by this act (as evidenced by his vision of Banquo's ghost) he is quick to plot further murders, especially after the witches conjure up a series of visions that warn him of the threat posed by Macduff. After he has Macduff's family murdered, he seems to have completely lost his moral compass. By the end of the play (or at least before the climactic final battle) this once valiant and noble thane has become a murderous, bloody monster.