Exactly. In my opinion the balance of the Force means that there is equal number of jedi and sith, in this case two jedi, Yoda and Obi-Wan, and two sith, Darth Sidious and Darth Vader.
yes - anakin/vader caused the death of most of the Jedi, then 20/25 years later he killed palpatine and the vader part of himself, thus destroying most of the Sith = balance