When topics like this come up, I like to go back to the beginning. Yes, the PT can color things, but when you look at some of the oldest materials (the earlier drafts of the screenplay, the novelizations, etc.), I have found that Lucas has been remarkably consistent on some points. We’ve always known that Vader was consumed by the dark side. How was left to the PT, but the how is immaterial to the outcome of the OT. Vader was redeemed. He came back. In that one act Vader/Anakin proves that Yoda was not telling Luke the whole truth. Yoda did say the dark side would forever dominate your destiny, and we don’t get to see what the redeemed Anakin might have become, but that he came back from the power that had consumed him really proves that the dark side is not what the Jedi think it is. They avoid it. They have no training in it. They really don’t know what it is except that when someone is consumed by it they become evil. They are closed to any exploration of it. When you have a natural force and you block all study of a part of it, that part with be very tempting to some. And rather than train Jedi to avoid temptation, they are trained to avoid anything that could lead to temptation. That was inherent to the advice Yoda gave in TESB and more explicitly detailed in the PT.

To me bringing balance to the force meant the destruction of all who had slanted teachings. Rather than the polar opposites, the force seeks those who delve into its mysteries to tread wisely but explore it fully. Not the light or dark, but the gray.