I don’t understand how a memory potion affects darkness in one’s soul. Like not remembering you’ve done something bad completely takes away the impulse to do bad things? It means your soul is turned bright white again? This seems pretty nonsensical to me. Why not just give all the villains memory potions then!
I think it makes sense that the memory potion affects the darkness in a person. We are the sum of our experiences. If a potion could erase ALL of a person’s memories he would become a tabula rasa – he would be as innocent as a newborn. So removing a person’s first step toward darkness should give them a second chance. This is a theme that has been addressed in many works, from John Locke to science fiction stories like Babylon 5 that deal with the “Death of Personality” which consists of wiping a person’s memories as punishment for their crimes.
I have more trouble with the question of whether it was a retcon of Bae’s character to have him want to kill Beowulf to begin with. To be fair, we should remember that we’ve only seen a few minutes of Bae’s character before the events of this episode – when Rumple first became the Dark One in season one’s “Desperate Souls.” Every scene of Bae after that episode would have taken place after he drank the potion.
Despite these questions, I think Rumple’s willingness to sacrifice Bae’s opinion of him in order to protect the boy was the best example of Rumple’s humanity that we have seen since the flashbacks to Rumple crippling himself in “Manhattan” during the second season.