the force is balanced with the genocide of the jedi in episode 3. you have two sith (there was always two sith) and two jedi (was lots and lots of them before). the gonocide leaves darth vader and sidious against yoda and obi wan.
the death of obi wan sees luke emerge as a jedi master which keeps the balance. yoda's death leaves the force unbalanced or maybe not. at this point darth vader is trapped in two minds. he is almost between the two sides (luke is having the same transition). this culminates in his killing of the emperor. his death leaves luke. he is not the jedi of old. he is capable of anger and hate as seen in the way he fights his father. but at the same time he is still a jedi and is able to control his dark side.
the genocide starts the balance and it is continued throughout the original films.