But wait....if the Force is a zero sum game (Every sith = -1 and every jedi = +1) then wouldn't the Sith be necessary to balance the force?
It isn't zero sum in that way. More like 'how does the river flow?'
Sith build irrigation systems, burms, and generally force the river into doing what they want (including flooding people they don't like). Jedi just drift along and go where the river takes 'em and trusts that the river is providing what is needed wherever it floods/irrigates/etc.
Sith, over time, twist how they use said river, and the river is trying constantly to get back to the "natural state" it was before the Sith started meddling. "Balance" just means "back to normal." Sith want a new balance on their terms.
Uhm. I didn't mean to build a case for the Sith... but, well, that's what it comes down to. Forcing/perverting the Force versus commingling/cooperating with it.