Stay on topic, please.
And Lucas can't be wrong; he gets to define what is right. Anfd Luke did not defeat 'all the Dark Side'. At most he defeatred two people who happened to use the Dar Side, and in fact he did not do the defeating personally. The entire point of it all is that the Dark Side may SEEM to give you more power- Quicker, Easier, More Seductive- it is in fact a barren path because it will eventually destroy you, as it did the Sith. Only the symbiotic Light Side can truly endure. Symbiosis vs. Parasitism. Lucas' theme with the Force.
Moving.
See, the Darkside is stronger due to the imbalance of the force, which has been caused by the Sith. Their dark magics corrupt the darkside further, and shift the balance of power in their favor. That is why Anakin was created, it was his destiny one way or another to defeat Sidious and free the galaxy. He could have done it in Episode 3, but as we all know that would not be. The Will of the Force created Anakin and left him when he turned darkside. This is the only reason Yoda and Obi-Wan survived, the will of the force was for them to train Luke so he could convert Vader to Anakin.
I am not sure it was the will of the Force, as such. I think the Force just created the possibility. The decision about how that greanted power should be used, and to what end, was down to Anakin. He had a destiny; he could choose hether to fulfill it or not. So the decision was not of the Force, but of man.