Macbeth has long been a subject of conflict for those who would label him as a tragic hero - a victim of circumstance and those who would call him the personification of evil. Whilst he is initially shocked by the witches prophesies and Banquo seems more excited:
Why do you start, and seem to fear
Things that do sound so fair?" (I.iii.51-52)
it does not take Macbeth long to accept his fate as it were. It is Macbeth’s internal struggle and the eventual victory of evil in his downfall that is central to Macbeth.
In real life, people face the same difficulties every day. We have some who are downright evil with no motivating factors apparent.Then we have those whose ambition is the driving force and they are the people who are most affected by the distinction between right and wrong, good for good's sake and the pursuit of ambition at all cost.
Manipulation is a powerful tool and, in Macbeth, the witches foresaw something that was believable and desirable to Macbeth. Taking his fate into his own hands, and goaded on by his overambitious wife made it easy for Macbeth to pursue his goals at all cost.
Some would say that Lady Macbeth was actually responsible for Macbeth's downfall and moral decay. However, it is interesting to note that whilst Lady Macbeth became increasingly haunted by her bad deeds, to the point of it making her insane, as her doctor put it
More needs she the divine than the physician...(V v 72)
becomes increasingly murderous.
and is still boastful.
Free will is important and Macbeth makes the decision to go forward with his wicked plan - even when it is clear that the witches prophesies are not going to be fulfilled.
I will not yield to kiss the ground before...(V viii 28)
This reinforces the view that he could have stopped at any point and accepted the consequences of his actions but he CHOSE not to.
Refer to the eNotes study guides, summaries and so on for explanations and insights into Macbeth.