I have a theory on Darth Vader on why there was still a little bit of good in him that Luke could sense. Basically I feel that when Vader was in that chamber that he went into where he could have his mask off and still survive was a place where Anakin lived because it was at a point where his mask was off and he wasn't Vader he was his former self. I feel during the times he was in there he regretted the things he has done but had no other choice but to do. And I feel he reflected on his biggest regret that he killed the love of his life Padme. And his love for Padme and his regret for her death was the part of him that held on to his good side. I hope I explained this well and is easy to understand. Thoughts on it?