why do the geese and sheep come forward to confess crimes when they know that the other criminals have been excuted?

Their confessions are a form of rebellion. One of the seven commandments states that "no animal shall kill any other animal." Napoleon's violation of this commandment is a sign to the other animals that they are not working together for the good of everyone and that he is indeed taking over the farm as dictator. Forcing the geese and sheep to confess crimes they haven't committed turns the political arena from communism to a fascist dictatorship. All of this stays in tune with Stalin's dealings in Russia.