Hamlet knows that he is not God and that taking life is a mortal sin. He knew that killing his uncle didn't mean much good for him, neither in this life nor in the afterlife. In his current life, he probably would have been severely punished for murder. Another thing to consider is that Hamlet would kill a king. Regicide was considered the worst crime a person could commit in Elizabethan times. No one at the time would have looked at Hamlet with more respect. Having a murder on his record would not have allowed him to go to the afterlife either. Even though it was just revenge for killing his dying father, the Elizabethans considered revenge a sin. Hamlet has clearly never killed anyone in the past and therefore tries to do it in the smartest way possible. Hamlet takes so long to deal with Claudius because he wants to avoid getting blood on his hands; he doesn't want to kill his uncle directly. This is why he creates the play within a play, hoping that Claudius will confess to the murder of King Hamlet so that he does not have to deal with Claudius.