By all accounts, Macbeth was a hero before the witches made their prophecies. He might even have remained a hero, not acting on his urges, if the prophecies had not begun to come true.
In Act I, Macbeth is described as truly heroic. Indeed, this is the reason why he gets promoted after defeating Macdonwald in battle.
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,(20)
Like valor's minion carved out his passage
Till he faced the slave. (Act I, Scene II, enotes pdf p. 9)
We know that Macbeth was not fighting solely to become the new Thane of Cawdor, because of his reaction when he is told he has the title.
The Thane of Cawdor lives. Why do you dress me
In borrow'd robes?(115) (p. 15)
Macbeth seems to respond initially to the prophecies the same way that Banquo does. The two men laugh and joke. Yet when he realizes that one of the prophecies has come true, his demeanor changes. We can actually see the breakdown of his character as he begins talking in asides, commenting on his ambitions.
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme! (p. 15)
Macbeth is already preparing himself to be king mentally. At first, he assumes he will not have to do anything.
If chance will have me king, why, chance(155)
may crown me
Without my stir. (p. 16)
He figures he has the kingship, and will not have to do anything to get it.
When he learns that he has not been named Malcolm’s successor, we see his true colors.
The Prince of Cumberland! That is a step(55)
On which I must fall down, or else o'erleap,
For in my way it lies. (p. 18)
Notice also that Macbeth does not want anyone to know about his ambition, and asks the stars to “hide your fires” so others cannot see “my black and deep desires” (p. 18). Macbeth wants to be king. He will stop at nothing to get it.
Macbeth does hesitate before killing Duncan. He is a coward at heart. Though he is ambitious, he finds any excuse not to do the bloody deed. Lady Macbeth urges him on though.
By the end, Macbeth has surely lost it. When things don’t seem to be going his way he returns to the witches. The new prophecies unsettle him even more than the first ones, and he acts recklessly. He assumes he can’t be killed my Macbeth because he is “of a woman born” (p. 60). He also assumes he is safe because the wood cannot move, and when he gets reports of moving trees he believes them! That is how unsettled he is. He throws himself blindly into the battle and is easily defeated by Macduff.
For the full text, read here: http://www.enotes.com/macbeth-text