How do characters with flaws (Macbeth) help us to understand ideas of the text and ourselves?
Includes construction of the characters, how they alter, themes/ideas, values promoted and audience responses.
Along with the above answer, I think it is pivotal to understand that all literature is the expression of the human condition. That being said, part of the human condition is to strive for better and be overall ambitious. What is necessary to understand about ambition is that it needs to have a limit.
Unchecked ambition, which is what both Macbeth and Lady Macbeth are guilty of, is the desire to achieve goals to any extreme possible. We must contrast Banquo to Macbeth, in this respect. Both Macbeth and Banquo heard the same prophecies from the witches, yet Banquo did not take any sort of action to ensure the prophecy that he would bear kings would come true. In essence, he could have attempted to have more children, or to also begin a line of murder, but he just heard the prophecy. Macbeth, on the other hand, according to Banquo, took the prophecies into his own hands to make certain that they would come true.
Thou hast it now: King, Cawdor, Glamis, all, As the weird women promised, and I fear Thou play'dst most foully for't:
What especially demonstrates the darkness of human nature is that it has been set up so that absolutely anyone is capable of absolutely anything given the correct circumstances. Macbeth was not a "murderer," in the non-military sense; he was actually a respected, and sensible man. A man who loved the king as "kinsmen" and "subject," but the desire for power drove him across the edge. We see a good man dynamically morph into a murderer with no feelings of regret or dismay -- showing us that any type of power can lead to corruption.
The goal of tragedy is to show us that our greatest qualities can undo us. Not only that, but tragedy shows us how fast we can be undone: one bad decision compounds the next, and within a week, we are at the top of the world and then dead. The tragic arch is a rapid ascent toward greatness and then a rapid descent toward death.
Aristotle says we must feel katharsis toward tragic heroes: we feel both pity and fear their fate as our own. We pity that a valiant, courageous man of action like Macbeth can turn into a blood-thirsty monster overnight. We fear that, by doing the same, his fate could befall us. We too might commit a crime in the pursuit of our ambitious goals ("the ends justifies the means"). This goal may blind us to morality, our loved ones, and our identities.
Lady Macbeth's death shows us that women may have had no chances to be great. Their status was so tied to their husbands' that they were essentially powerless--influential only behind closed doors. Macbeth is a great warrior, but the best Lady Macbeth can be is a good hostess. We pity her death and madness because her ambition may have been greater than her husband's, for she was trying to defy her gender limitations.