Hamlet's father's ghost tells Hamlet that his brother, Claudius, murdered him. He tells Hamlet that, if he doesn't seek revenge, he is dull and lazy.
Well... I guess I'm lazy then! I suppose I'll think about it...
Hamlet decides that he should avenge his father's death by killing Claudius. He sees Claudius confessing his sin and decides to wait until he is sinning to kill him.
I should wait and kill him when he's sinning so he'll go to hell!
Forgive me Father, for I have sinned!
Hamlet tells his mother that Claudius killed his father. He then hears a voice behind a tapestry and assumes it is Claudius. He stabs him through the tapestry, but it wasn't Claudius. He killed Polonius instead!
Oopsie! I thought Polonius was Claudius! I've gone mad by the way!