If murdering a member of your family means you shouldn’t get to be redeemed then Anakin/Vader shouldn’t have been. He murdered his very pregnant wife by strangling her with the dark side of the force.
Anakin/Vader didn’t kill Padme. If he had killed her, or Luke, or Leia, especially if they were trying to help him back to the Light, then yes, that would have been the end of any potential redemption. That’s why Vader thought he was beyond redemption, because Sidious told him he killed her.
Kylo was offered a clear cut choice by the man who raised him and loved him. There was nothing forcing Kylo to stay on the Dark Side, but he chose it anyway. Unless he was secretly playing double agent and this was all part of some bigger plan, then that was it. He had made his choice.
Then he made the same choice again in TLJ, after Snoke was dead and he had no reason to stay beyond personal ambition.