What is a good sentence showing the overconfidence of Macbeth?
Macbeth is consumed with anxiety as well as confidence, a balance of opposites. However, towards the end of the play, his overconfidence increases and his fear diminishes. We can blame this fatal flaw on his increasing confidence but it is really the result of him becoming more and more delusional. Note that before he kills Duncan, he is quite apprehensive. His superego (conscience) and his id (desire) are battling in his own mind. He feels anxiety and potential guilt but his desire (with Lady Macbeth's encouragement) eventually overrules his more reasonable side. After killing Duncan, that anxiety never really goes away, but he does become more confident in his determination to protect his power and position as king. As his greed grows, so does his confidence.
Macbeth's delusional confidence reaches the pinnacle in Act 5. With his simplistic and overconfident interpretations of the witches' latest messages, he feels unstoppable. In Act 5, Scene 1, he repeats that he has nothing to worry about unless Birnam Wood comes to his castle. He can not conceive of a forest coming to attack him, so he feels there is nothing to worry about. He doesn't take the time to consider how these messages might be interpreted in other ways. When he is informed that thousands of soldiers are marching against him, he says, "Bring it after me. / I will not be afraid of death and bane. / Till Birnam Forest come to Dunsinane." (V.iii.67-69) Even though the odds are increasingly against him, he basically says, "Bring it on."
In the next scene, even though some of his own men have deserted him, he is resolute. He says "Our castle's strength will laugh a siege to scorn." (V.iv.3)