Personally, I've always had problems with how Vader's 'redemption' in RotJ is handled, and I'm not alone - it's the "does the stormtrooper hit his head?" of serious moral questions with the OT...
Whatever nobility there may be in Vader's decision to save Luke, and even if it was a considered decision to throw the Emperor down the latest and best Conveniently Placed Big Hole built by the GFFA's engineers, rather than simply something he had to do to save Luke... how does that atone for all the crimes he's committed, or for those he stood by and watched with grim satisfaction, like the destruction of Alderaan?
To understand why, I suggest, we have to look at why Anakin Skywalker became Darth Vader. Now we have no idea yet of the details of this, but I hope that it involves a situation where he has to make a choice for the greater good: for instance, he might have to hold off from rescuing Padmé in order to save the newly-established Empire from chaos.
Now consider what this would do to a Jedi - how can he reconcile serving the Light Side with causing that sort of suffering? Short answer, human answer - he can't.
So he can't be human and Light, but he can't not be human; so he slips, slides, and gives himself up to anger and hate. Because he let what happened happen; and also because he feels that it was wrong to do that, in spite of what he achieved by doing so.
Palpatine probably planned all this; he probably even tells Anakin as much. It doesn't matter. Anakin crosses the line, he surrenders to the Dark Side. And if there is a strict duality between Light and Dark - as 25,000 years of Jedi teaching insist, and the Sith Master doesn't seem to dispute the point - then having crossed the line, he becomes doomed to damnation.
There is, apparently, no way back for Anakin Skywalker: he becomes Darth Vader.
But consider these quotes from RotJ...
Then, in the climactic duel, Luke gives in - according to the script - to 'anger' and 'hatred'... now - in spite of what the dialogue says - this *is* the Dark Side. What else is it?
Luke Skywalker, like his father before him, surrenders to the Dark Side.
And then, Vader watches as his son does what all Jedi dogma says is impossible.
He walks back out of the darkness.
Duality? Dark? Light? 'Once you start down the Dark path, forever will it dominate your destiny'?
Cr*p. Piffle. Rubbish. B*lls. Bullsh*t.
And with that, Anakin Skywalker breaks free of the mental shackles in which Qwi-Gon, Yoda, Ben, Mace and Palpatine had bound him, breaks free, and follows his son to victory.
Let's not pull any punches here - think about Alderaan, ten thousand Jedi dead, and 'little' things like Ozzel, Needa, Han-in-Carbonite and Leia's mind-probe... but the buck doesn't stop with Vader - it goes right back to the Jedi Order and their dogma.
Vader isn't a living embodiment of darkness, whatever he or anyone else thought - he's a man, Anakin Skywalker, flawed and human, but with enough good and strength in him to throw an old megalomaniac down a big hole in the floor.
After a life lived as a slave - to Toydarians, prophecies, Jedi, Sith, and a dualistic doctrine of Light and Dark which damned him... he dies free. Is he 'redeemed'? Does he 'return to the Light'? I don't know - but what should be obvious is that, in spite of everything...
DARTH VADER IS NOT DAMNED!
And at last, as he dies, he knows it...