I believe the correct answer is: his inability to act.
In William Shakespeare’s drama “Hamlet”, the main character Hamlet has a tragic flaw which will undoubtedly lead to his tragic end. Hamlet's tragic flaw is his inability to act, which is why he is indecisive throughout whole drama. He can’t decide if he should act and when he should act to take revenge for his father's death. This leads him and many others (except Horatio) to their graves.