By strict definition, there is a type of play known as a "morality play" that was popular during the Middle Ages. Certainly this kind of play influenced the dramatic movement which was to become so popular in England during Shakespeare's time. And while there were other playwrights of that time, ostensibly Shakespeare has been the most enduring and influential.
A morality play was one that taught morals. It was rooted in theological values, though it did not use stories from the Bible to do so. The Roman Catholic Church exerted enormous power at the time when there was really no distinction between church and state; it was through these plays that the concepts of good vs. evil were presented to those who viewing the play, thus encouraging moral behavior. Often times presented on the backs of wagons in a village square (long before theaters). The plays...
...were more concerned with morality than spirit.
The struggle of morality revolved around evil and good battling over the "soul."
It is probably safe to assume that this kind of dramatic tradition affected not only Shakespeare, but all playwrights of his time, for this form of play was the "predecessor" of what became the dramatic movement of Renaissance England.
Is Macbeth one of Shakespeare's moral plays? Had we been discussing Hamlet, it would be an easy answer, for Hamlet himself, charged by his dead father's ghost to avenge his murder, asks himself if the ghost is truly that of his father, or a spirit bent on causing Hamlet to commit a mortal sin (killing a king) and losing his eternal soul.
In Macbeth, however, we need to look more closely. A major point is that Macbeth starts out as a brave and much-loved soldier for King Duncan, who is also Macbeth's cousin and friend.
Moral is defined as...
...expressing or conveying truths or counsel as to right conduct, as a speaker or a literary work...
Macbeth is riddled with what is generally called "vaulting ambition." This is his "tragic flaw."
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself... (I.vii.25-27)
He finds that he really does not have the desire to kill Duncan, at least not for the sake of money or politics, but his ambition will not allow him to be satisfied with what he does have—Duncan has greatly favored and rewarded him.
Macbeth is also squeamish with what he has done in killing Duncan:
I am afraid to think what I have done
Look on't again I dare not. (II.ii.65-66)
This indicates that it does not sit well with Macbeth's basic nature; but as he continues to kill, it bothers him less until he knows there is no turning back. Macbeth is a conflicted man: he wants to be King for the power and position, but is does not naturally come by the treachery that it takes to take Duncan's life. It is Lady Macbeth's constant badgering and insults that drive Macbeth's sense of purpose.
If this were my assignment, I would write something like:
William Shakespeare's Macbeth tells the story of a seemingly moral man who loses his soul due to his treachery—not because it comes naturally to him, but because of an inescapable desire to be king.
I would write about Macbeth's moral character at the start (his valor, the arguments he gives his wife for not killing, etc.), his guilt afterward and how he deteriorates as a moral person in the rest of the play.