as for balance to the force, you seem to forget that the force isn't just in jedi's and sith. It in that tree, this rock, the air around us, and let's not forget all of the hutt's rancor's, dionaga's, and general all around jack asses in the galaxy. there is already plenty of force flowing through some bad stuff in the universe.. flowing through a sith as well puts it out of balance.
pretty simple, eh?