Everyone needs to know the real story of Claudius
Hamlet's fatal flaw was how indecisive he was. He knows that Claudius is the one who killed his father and he wants revenge.
After the ghost of his father visits him, to tell him that Claudius is the one who killed him, Hamlet seeks revenge. Hamlet was very prideful when he was successful with his revenge plan by killing Claudius.
I know what you did
Started off as a powerful prince and was loved as everyone and then he died and everyone thought he had gone crazy.
He realizes that he is at fault for everyones deaths.
It's all my fault...
Hamlet's nemesis is his uncle Claudius because he is the one who is guilty of murdering Hamlet's father. Also, after killing Hamlet's father he married Hamlet's mother to make himself king.
Hamlet was struggling with grief because he felt that he was responsible for all of the deaths that occurred.