Hamlet does have a fatal flaw. He is emotionally inconsistent in matters. First, He is hesitant in believing the truth about his Uncle Claudius. After speaking with his father's ghost, Hamlet learns of his father's murderer. While it is an apparition, it appears to be Hamlet's father who is telling young Hamlet of how Claudius murdered him. Hamlet is not convinced. He sets up a role play, a drama, to reenact his father's death. Once convinced that Claudius has indeed murdered his father, Hamlet is slow to act in his revenge:
Slow to the conviction that the ghost is his dead father and that Claudius is guilty of regicide, Hamlet does not go straight to the task at hand. Hamlet's delay or procrastination is something about which critics have wondered and that the character himself agonizes, his self-reproach reaching an apex in Act IV, scene iv, which concludes with the words "O, from this time forth, My thoughts be bloody, or be nothing worth!" (lines 65-66). The question remains: Why doesn't Hamlet act?
Also, Hamlet is inconsistent in his feelings for Ophelia. He seems to be in love with her. Then, again, he isn't in love with her. He tells her he doesn't care about her, but at her funeral, he seems to be in love with her. Again, Hamlet is inconsistent. This is his fatal flaw.
Also, Hamlet is not careful with his emotions. He senses that someone is eavesdropping on the conversation he has had with his mother. Thinking it is Claudius, he kills Polonius, Ophelia's father. Instead of being rational, Hamlet appears irrational. He murders the wrong man because he is an emotional wreck. He does not have control of his emotions.
Truly, Hamlet is emotionally unstable. He has bouts of depression. He is easily depressed. He sighs and sighs, and he gives in to melancholy emotions:
Hamlet is also a melancholy figure, given to depression, who is victimized by a cruel fate and compelled to undertake a revenge mission for which he is not prepared. Not only are Hamlet's musings about life extensive, they are uniformly dark. Seen in this light, Hamlet does not act because he lacks the emotional fortitude to do so, depression and courage being difficult to reconcile.
These inconsistent, irrational actions cause Hamlet to postpone Claudius' death which in turn leads to his own death. Had he killed Claudius sooner, perhaps Claudius would not have had time to plan Hamlet's death by Laertes. Since Hamlet procrastinated, he gave Claudius time to plan to murder him. Hamlet falls because of his emotional inconsistency, his fatal flaw.