Choice
A bit of a random idea.
Vader never knew what the Light was, only that it cleansed every darkness in one's soul. The Force had no mercy. While it pined for balance, it demanded none of it in the end. There was only darkness and light. Vader chose power… he chose Padme.
Yet in the end, Luke tried his very best to save him. to turn him to the Light. Whatever was left of Anakin was gone long before. However, for all of that – Luke was still his son - Padme's son.
She would never want him to harm their son.
For her, he chose Luke.