My good friend, Granny-Wan, found this video recently and it really made me think about the scene in ROTJ when Palps is using Sith Lightning on Luke and Vader's reaction.

As we see Vader watching Palps slowly electrify his son, what was going through his head?

Just the angle at which (brilliantly by Dave Prowse, I might add) Vader looks back and forth between Palps and Luke demonstrates absolutely that he is torn between his new found son and the power of his hated Master. He's actually always looked sad to me in that scene even though you can't see any facial expressions. I always found that interesting.

Could he dig deep down into himself and find the long-lost love that he once felt for his unborn child before the terrible events occurred that brought him down and sent him finally down the dark path?

I think he truly might have been replaying in his mind those events that started him down the path to the Dark Side. How would he not have let Padme's memory slip into his mind as he watched their son being fried like bacon? Surely the tenderness he had felt toward her came rushing back during those moments and allowed him to realize that the good that was once inside of him was really still there. He had buried it for decades deep within himself under all of the hatred and anger that he had allowed to rule his life, but he found it once again during those pivotal moments.

I wonder that he wouldn't have found the strength and love to save Luke earlier than he did. It was obvious that he wanted out from underneath Palps thumb before those moments, but his connection with the Dark Side was so strong that it must have taken all of his will to rip himself from that grip and embrace the light that was love for his son.

What a painful yet inspirational moment all wrapped up in one.