Macbeth is described as a tragic hero and therefore to meet this definition he would have to show signs of morality and be more affected by his circumstances and external factors than his own evil (or not so evil)character.
Duncan awards Macbeth for his bravery on the battlefield and he becomes the Thane of Cawdor. This is the start really of Macbeth's obsession with the witches' prophesies as he feels they must be right, having accurately predicted the first stage of his achievement. His ambition and trust in his manipulative wife also contribute to his downfall.
Macbeth is easily confused and, after his first encounter with the witches cannot determine whether their motives are for good or not. He does have a moral compass.
He then begins to doubt reality itself as he states that
But what it is not (I.iii.141-142)
Macbeth begins to doubt his intentions when he is on the brink of killing Duncan
We will proceed no further in this business.
He hath honour'd of me of late...(I.vii.30-31)
and so he does show a sense of morality- although easily overriden. He is haunted by Banquo's ghost and his hallucinations and inablity to sleep when he is commanded to
sleep no more (II.ii.37)
are symptoms of his unease. He knows it is wrong but he is so driven by witches' prophesies (he even revisits them to give him courage to proceed) and Lady Macbeth's encouragement that it seems secondary. Lady Macbeth however is no match for the witches and Macbeth even believes he is invincible and cannot control the situation because of the witches' words.
It does not matter towards the end, when he can see that things are failing and he will ultimately be defeated because the witches said he would triumph. It is out of his hands.
The mind I sway by, and the heart I bear
Shall never sag with doubt, nor shake with fear(V.iii.9-10).
Again, he is suggesting that, despite knowing what is going on he cannot show fear or remorse. He was awarded for valour when he was made Thane of cawdor so he is equating this battle in the same way. Somehow to him, it is the valiant and 'right' thing to do.