The scene "We need to find the source" where Eleven says

"Billy. He knows it. He's been there. To the source."

And then she's taken to the beach. She sees Neil and billy's mom and little billy. She sees Billy's mom abandoning her son and all hell breaking loose.

What if that is the source. What if Billy's nameless mother is the Mindflayer, or part of it.

And by some twisted logic she wants her son back.

Or, both Billy's parents are the source. The two ancient restless tortures collide : Neil, the blind rage and generational anger, and Billy's mother, the woman who abandons her child. And Billy's caught up in between.

Gif by @suledins

Steve : getting bitchy about not receiving attention

"We need to talk."

"But you came back to me now, right? .. You came back to me?"

And

Steve (dreamily) : Damn ..

Nancy : What?