A better “hypothesis” (a theory is something you prove after you make a “hypothesis” - remember your science people) might be, that The Bride fell in love with someone else, and wanted out of that life of being an assassin. That is why she left. The child may or may not be Bill’s. She might have just said that in hope that Bill would spare her life and the life of her child.
I saw where the bullet hit her in the head. It wasn’t a killing shot. He did not shoot her in between the eyes. He shot her along her left temple, suggesting that maybe he had better keep her alive just a little longer, just so her child was born. So if he was the father, he could claim the child as his own.
Maybe, he had designs to raise the Bride’s child as an assassin, but she did not want that kind of life for her child. OR maybe Bill was hoping that by taking her child, the Bride might come to her senses and return to the fold. Only he did not count on her being in a coma for four years, being raped while in that coma, and waking up filled with rage, rage so deep as to make her go cold where killing was the only way imagined. To take revenge in the manner that she is doing, you would have to go cold inside. You would have to have no feeling what so ever. To serve out death, there could be no warm sympathy. Thus the Klingon Proverb. To serve revenge, you cannot have any warmth toward your enemies.
Well GUESS WHAT BILL. There is nothing more furocious than a female protecting her young.
So serve up Part Two with gusto.