Board Thread:Character Discussion/@comment-31564770-20170322233001/@comment-25926288-20170322234417

Cool theory. I agree with most of the theory as it doesn't go against canon. I view the serum creations as a stepped method. As we know the serum could work on a shared body, later separating the two. So, from the facts given, I theorize that the serum simply projects the "passionate, darker impulses" of their heart from itself (or as Jekyll says "the primitive beast" from the higher, more nobler aspects"). If we go with some Freudian terms, it separates the Id and Superego from one another. If this is the case, then with the serum divides the two, it gives the Id corporeal form, while the Superego (and Ego) stays within.

However, I don't think much was put into than "Evil parts are out, but the original is still the anchor for life". Given Hyde and Jekyll were willing to do evil things, I'd wager is because they are still linked. (It's not like Regina's heat was any less dark when she ripped it out of Queenie (which was a projection of her own heart which she later ripped out to save Zelena and it was still just as dark as always).

Long story short, I think the serum counterparts are simply a projection of the "primal beast" within given corporeal form. Ex. Queenie is just a magicked form representing the darkness within Regina. Thus the only way to kill Queenie is to eliminate the darkness within Regina, either through magic or death.