The major flaw in Hamlet's character is his tendency to hesitate and overthink.
Throughout most of the play, Hamlet is troubled by his tendency to overthink, which is also what leads him to fail. Although he finds out that Claudius murdered his father at the beginning of the play, he delays the act of killing his uncle in order to get revenge every time he is strongly influenced by his own thoughts. In other words, every time Hamlet finds an opportunity to murder Claudius, he starts to hesitate about it. Moreover, Hamlet's tendency to overthink is also present in his famous soliloquy "To be or not to be", where he vacillates between life and death.
