Ever since
TLJ came out, there's been a reemergence of discussions about the light side, the dark side and the balance between them. Some claim the ST is retconning the established lore, some say the dark side is a cancer on the Force, the imbalance itself.
In order to keep the discussion clean, I wanted to share a couple of quotes regarding the balance between the light and dark from George Lucas himself. They are his comments taken throughout the years, from the 80s to 2011-ish, showing a consistency of the concept in his mind. I hope they'll reveal the true nature of the balance of the Force in the eyes of the creator himself.
"The overriding philosophy in Episode I—and in all the Star Wars movies, for that matter—is the balance between good and evil." -George Lucas, quoted in L. Bouzereau, Star Wars: The Making of Episode I, 1999
"In each of us we to have balance these emotions, and in the Star Wars saga the most important point is balance, balance between everything." -George Lucas, Time Magazine article, 2002
"The idea of positive and negative, that there are two sides to an entity, a push and a pull, a yin and a yang, and the struggle between the two sides are issues of nature that I wanted to include in the film." -George Lucas, quoted in L. Bouzereau, Star Wars: The Annotated Screenplays
"The Force has two sides - [Light and Dark]. It is not a[n inherently] malevolent or a benevolent thing. It has a bad side to it, involving hate and fear, and it has a good side, involving love, charity, fairness and hope." -George Lucas, Times Magazine, 1980
"I wanted to have this mythological footing because I was basing the films on the idea that the Force has two sides, the good side, the evil side, and they both need to be there. Most religions are built on that, whether it's called yin and yang, God and the devil—everything is built on the push-pull tension created by two sides of the equation. Right from the very beginning, that was the key issue in Star Wars." -George Lucas, Times Magazine, 2002
The one below has been often quoted as a Word of God regarding the Dark Side being a cancer of the Force. It's important to note, that it's misinterpreted. George comments here on the Sith and their relationship with the Force and each other, comparing it to a cancerous relationship.
It says nothing about the Dark Side itself - just people who embrace it completely and discard the light.
"So the idea of temptation is one of the things we struggle against, and the temptation obviously is the temptation to go to the dark side. One of the themes throughout the films is that the Sith lords, when they started out thousands of years ago, embraced the dark side.
They were greedy and self-centered and they all wanted to take over, so they killed each other. Eventually, there was only one left, and that one took on an apprentice. And for thousands of years, the master would teach the apprentice, the master would die, the apprentice would then teach another apprentice, become the master, and so on.
But there could never be any more than two of them, because if there were, they would try to get rid of the leader, which is exactly what Vader was trying to do, and that's exactly what the Emperor was trying to do. The Emperor was trying to get rid of Vader, and Vader was trying to get rid of the Emperor.
And that is the antithesis of a symbiotic relationship, in which if you do that, you become cancer, and you eventually kill the host, and everything dies." -George Lucas, TIME magazine, April 26, 1999
And finally, a very important quote from the Mortis arc of the Clone Wars cartoon, in which George was hugely involved:
"It is only here that I can control them. A family in balance. The light and the dark. Day with night. Destruction, replaced by creation...Too much light or dark would be the undoing of life as you understand it."
You may call it the final words of George Lucas on the nature of the Force - and I believe its consistent with everything we've seen since 1977.