What would be an argumentative essay about Macbeth's tragic flaw?

In an argumentative essay about Macbeth's tragic flaw, you first need to determine what that tragic flaw is and build a thesis. I'm going to argue that his ambition is a good choice, but there are certainly other options you could use. In your introduction, you should provide the title and author of the play, a brief summary of the play as it relates to ambition (or whatever trait you choose), and then the thesis, which in this case would be "Macbeth's tragic flaw is his ambition, which eventually leads to his downfall."
Then you'll need to look for supporting evidence from the play. Three good examples could become your three body paragraphs, with thorough explanations of how each example leads to his downfall. Here are some possible examples for Macbeth's ambition being a tragic flaw:
1) Macbeth is willing to let some witches determine his course of action because of his ambitions to have power. This exchange happens in the first act to guide his decisions:




SECOND WITCH
All hail, Macbeth! Hail to thee, thane of Cawdor!






SECOND WITCH
All hail, Macbeth! Hail to you, Thane of Cawdor!






THIRD WITCH
All hail, Macbeth, that shalt be king hereafter! (I.iii.52–53)

At first, Macbeth thinks the entire idea is crazy; after all, he reasons, "But how of Cawdor? The thane of Cawdor lives, / A prosperous gentleman, and to be king / Stands not within the prospect of belief, / No more than to be Cawdor" (I.iii.76–79). However, the more he considers it, the better it sounds, and it begins to shape the actions he is willing to take—to the point of murder. Which leads us to the next point. . . .
2) Macbeth is willing to commit murder in the name of his ambition. When Lady Macbeth insults his manhood for being too slow to act against Duncan, Macbeth allows himself to be manipulated by her words. How does she push him? She knows that his ambition is the key to motivating him. She says, "Wouldst thou have that / Which thou esteem’st the ornament of life" (I.vii.47–48), reminding him that this is his destiny, as he sees it. This is the thing he's always longed for. And now is the moment to act. She also tells him, "And to be more than what you were, you would / Be so much more the man" (I.vii.54–58). Again, she's calling to Macbeth's mind the idea that he could be something so much greater.
3) Macbeth's ambition leads him to misinterpret the warnings in act 4. Beware Macduff. No man born of a woman can hurt him. Macbeth can never be beaten until Great Birnam Wood comes to Dunsinane Hill. Macbeth's ambition makes all of these warnings seem like impossibilities, and he cannot see the truth playing out in front of him. Even given the warnings, he proceeds with his plans. He hears what he wants and believes what he wants—because he has ambitious goals. In fact, he immediately says, "That will never be" (IV.i.104).
Macbeth's ambition is one of his tragic flaws, and ultimately, it destroys him.