It's the same person. This isn't a universe where every human exists in this primitive state and they all have to do terrible things to survive but some are less worse than others. Anakin fell to the dark side because that's who he is. The capacity was there. It's the same thing with Kylo Ren. Kylo Ren is a bad dude. A very, very bad dude. He, too, is an accomplice to genocide. He murdered Han. He knows these things are wrong yet he continues to do them. No redemption for him.
I hope you don't think I'm angling for a Kylo redemption, because I'm not.
All I'm saying is that RotJ puts in the work to show that yes, "Vader" and "Anakin" are distinct halves that were fighting for dominance. Obi-Wan wasn't just being cute when he said "From a certain point of view". Otherwise, Luke's appeals to Vader, his realization at his dark half (The cave on Dagobah? That was a hint towards Vader being Luke's dad
and a premonition that he is just as susceptible to becoming like him), his throwing away the lightsaber and refusing to kill his dad like Obi-Wan and Yoda wanted,
aka the entire Thrown Room scene doesn't work.
Because you're right, having a mass-murderer, torturing, machine monster be redeemed wouldn't fly.
But freeing the good half from the evil influence of the bad half can work. Burn away the suit to free the Force Ghost inside.
Now, whether or not that means redemption is
always possible no matter what is a different issue entirely. Kylo, for example, was a reverse of the situation, where his dark "Kylo" half was struggling against his lighter "Ben" half, and the "Kylo" part won out when he shishkebabed Han. Looking at it from a storytelling perspective it would make the most sense to keep him on this path.
