I personally just dislike the idea that Ben had to die to atone for his sins as Kylo. He was a young man. Should he feel regret about his choices for the rest of his life? Absolutely. But he should channel that into doing positive things for the galaxy.
Vader was a completely different circumstance. Living in his body was a living hell, and he had already spent the other half of his life trying to be a good person as a Jedi. He was also old enough for his death to not be so tragic.