I suppose Macbeth conveys something about humanity and/or about the human experience, but what makes the play great is the same thing that makes Shakespeare great: it is his poetry, his use of language, his genius, his eloquence. Macbeth contains some of the most beautiful poetry Shakespeare ever wrote. For example:
Methought I heard a voice cry "Sleep no more!
Macbeth does murder sleep:--the innocent sleep,
Sleep that knits up the raveled sleave of care,
The death of each day's life, sore labor's bath,
Balm of hurt minds, great nature's second course,
Chief nourisher in life's feast.
Still it cried "Sleep no more!" to all the house.
"Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more."
(Act 2, Scene 2)
It seems to me that it is marvels such as this that should be pointed out to students rather than insights into human nature. Most humans don't behave like Lord and Lady Macbeth. While looking for insights into human nature or the human experience, readers can overlook the beautiful language that is staring at them on every page. Here is another example to savor:
We have scorched the snake, not killed it.
She'll close and be herself whilst our poor malice
Remains in danger of her former tooth.
But let the frame of things disjoint, both the worlds suffer,
Ere we will eat our meal in fear, and sleep
In the affliction of these terrible dreams
That shake us nightly. Better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than on the torture of the mind to lie
In restless ecstasy. Duncan is in his grave.
After life's fitful fever he sleeps well.
Treason has done his worst; nor steel nor poison,
Malice domestic, foreign levy, nothing
Can touch him further.
(Act 3, Scene 2)
Notice the alliteration of F sounds in "After life's fitful fever" and the suggestion that life is a sort of sickness which people recover from by dying.
And Macbeth to the Doctor who is attending his wife:
Canst thou not minister to a mind diseased,
Pluck from the memory a rooted sorrow,
Raze out the written troubles of the brain,
And with some sweet oblivious antidote
Cleanse the stuffed bosom of that perilous stuff
Which weighs upon the heart?
(Act 5, Scene 3)
(Pluck from the memory a rooted sorrow. If only it were possible!)
And finally when Seyton tells Macbeth his wife is dead:
She should have died hereafter.
There would have been a time for such a word.
Tomorrow and tomorrow and tomorrow
Creeps in this petty pace from day to day
To the last syllable of recorded time,
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
(Act 5, Scene 5)
Shakespeare would have prefered to be a poet, but he had to earn a living, so he became an actor and playwright. He put his poetry into the mouths of his characters.