Dustcan 
"[@Vivace":](/forums/generals/topics/star-wars-general?post_id=4755213#post_4755213
)
According to George Lucas the Light/Dark sides of the Force are less Yin/Yang and more Buddhist(?). They aren't complementing halves to a whole, the Darks side is a corrupting cancer on the Force. So excising the Dark side entirely would cleanse and balance the Force.
Hence destroying the Sith brings balance.
Edit:Sorta ninja'd
Dogs
According to George Lucas the Light/Dark sides of the Force are less Yin/Yang and more Buddhist(?). They aren't complementing halves to a whole, the Darks side is a corrupting cancer on the Force. So excising the Dark side entirely would cleanse and balance the Force.
Hence destroying the Sith brings balance.
Edit:Sorta ninja'd