The draw to Shakespeare's MacBeth can certainly be attributed to its darkness. Shakespeare masterfully exposes the weaknesses of human beings as the foundation for this tragic play.
In the beginning of Act One, the witches, " interpreted variously as custodians of evil", encircle the darkness of their cauldron summoning the dark spirits of MacBeth's future.
The Witches are summoned to leave, but they do not leave without stating that what is normally “fair” will be “foul,” and what is “foul” will be “fair.”
In the same beginning act, King Duncan calls for the execution of the Thane of Cawdor due to his betrayal. The act of a traitor portrays a dark side of human nature.
The dark shroud of ambition falls upon MacBeth, who turns to murdering King Duncan to ensure his crown. Betrayal, suspicion, and paranoia follow this criminal act, and MacBeth falls deeper into the grasp of darkness and evil. The overpowerment of guilt plays its toll on Lady MacBeth, who, unable to escape guilt's grip, takes her own life. The ultimate darkness, the brutal massacre of MacDuff's family, brings MacBeth and MacDuff to final blows and ultimately MacBeth's eternal darkness.
There are many ways of looking at this issue but you could certainly argue that one of the function of the witches was to identify the murderous, ambitious and evil thoughts that Macbeth was already having. Their "prophecy" was actually just a nudge to someone who was already contemplating such heinous crimes. This play also shows a man who may have started out as "good" but with one crime embarks on a cycle of murder and bloodletting. Giving into our "dark side" seems to be addictive. Blood will have blood.
Humanity has the benefit of free will. Every person inherently has the potential for good and the potential for evil. The fact that Lady Macbeth spurs her husband to murder a good King whom everyone loves (even the creatures of the earth rebel and act crazily when he is unjustly taken from them) in order to promote their own political advancement is just the beginning. Macbeth plays with everything that is "dark"...murder, manipulation, lies, deceit, disloyalty, and disobedience to the higher power (hence the reason Macbeth can't sleep or pray).
The play also indicates how once a chain of events is set in motion it becomes impossible to undo the past, and, in the case of Macbeth ond Lady Macbeth, leads inexorably to their deaths. As an audience we see the malevolent force of the witches and their suggestions. We witness also the slide of the brave, loyal and virtuous into the maelstrom of deceit and murder.
I agree with earlier suggestions about the fear cultivated in the audience of the ability for all to be swayed along such a destructive path when steered by ambition.
Macbeth's descent into evil recalls Joseph Conrad's "Heart of Darkness" and even Paradise Lost as Satan declares that it is better to rule in Hell than to serve any other. Certainly, evil way leads to evil way--"valuting ambition" leaps further and further into the darkness of Macbeth's heart. This theme of the commitment to evil once one has crossed into the preternatural world is what is most disturbing and fascinating both about Shakespeare's Macbeth, for
what a tangled web we weave
When first we practice to deceive
-- "Marmion" by Sir Walter Scott
Continuing the web metaphor, in his great novel, Notre Dame de Paris [The Hunchback of Notre Dame], the bishop, who delves in alchemy is warned by a friend that he is like the fly who tries to penetrate a web, but once snared, cannot go back. Macbeth is caught in the web of evil and can only descend further.
(Interestingly, in real life, it was the Scottish king Duncan who was the most evil--at least according to the tour guides at Inverness, Scotland.)
The previous posts have featured many great points of discussion. I think that being able to focus on how the play explores the very depths to which humans can descend and have this feel "entirely normal" at a particular point is what might be the most disturbing aspect of the play. I think that it is shown in a very strong and rich manner and this is what the play leaves us with about the composition of human nature. For me, the most scary part of the play and of Macbeth's character is not as much to what levels people can descend, but rather how "normal" it can feel after a certain point. Macbeth almost becomes accustomed to engaging in a morally depraved consciousness.
While it's true that Macbeth and Lady Macbeth commit some horrendous acts out of ambition and greed (among other things), what is most intriguing is how it all begins. Obviously the Macbeths had these characteristics in them or they would not have been so easily moved to act as they did; however, it is the prompting of the witches which begins their downward spiral.
One of the questions my students always ask is whether the witches had supernatural abilities or were just "messin' with his mind." I say both. They can create havoc for a sailor and sail in a sieve, but they can't kill him. They can be where Macbeth will be before he gets there, appear and disappear, and conjure apparitions, but they can't force him to act. We also know they're spiteful (the woman wouldn't share her snack on demand so the witch vowed to punish her husband).
We generally come full circle to this conclusion: Macbeth must have done something or been someone the witches didn't like. Recognizing his arrogance and "vaulting ambition," they seem to have set him up for a scenario where his own worst attributes would be his undoing.
If all that is true, perhaps it is the witches' selfish and vengeful spirits (no pun intended) which demonstrate the worst of human (or close to human) nature.
The darkness in human nature is, of course, best exemplified by Macbeth and Lady Macbeth. They lust after power, plot, deceive, and murder. All the while, they ignore any intrusive impulse toward human decency and convince themselves that they can live with their own indecencies.
Every despicable deed committed by Macbeth and his wife emanates from the deadliest of the seven deadly sins: the sin of pride. They exert their will to elevate themselves, defying all that is good and holy--defying the very will of God. Macbeth persists, even as he realizes he has given up his soul, his "eternal jewel."
Most of what is vile in human nature is demonstrated in the play, but the most profound darkness in human nature is pride, ego, hubris: My will be done.
One of the dark elements of human nature revealed in Macbeth is the subjugation of one's own self and morals for power. The three hags told Macbeth that he would be king. No doubt he believed them, but rather than wait and see how cirumstances would progress, he decided to help things along. It doesn't matter that Lady Macbeth seemed to be a primary force in this course of events. Macbeth chose to go along with it. He knew it was wrong, but he allowed it to happen.
Macbeth shows the extent to which people will go in order to realize their own ambitions, even to murder. As soon as it is suggested to Macbeth that he will be "king thereafter," the wheels begin turning in his subconscious. It is his insanely ambitious wife, however, who immediately seizes the prophecy and urges him on to murder. As heinous as the murders committed by Macbeth are in the play, it is Lady Macbeth who offers the most compelling character study, in my opinion. She is the epitome of the dark side of human nature, horrifying the viewer/reader when she states that she would rip a nursing child from her breast and dash out its brains if that's what it would take to show Macbeth her desire for his kingship and her willingness to help him secure it.