Here is my crackpot theory that tries to put the hood and magic factor into the picture. So it’s Emma’s dark non-savior side that she split off by drinking the serum. That half of Emma decided, whether because she is actually selfish and bad, or because she is still trying to save everyone, but has less ethical qualms about how to go about it, that she needs more power. She therefore stabs Rumple with the dagger and gets his powers (a logical, if depressing demise for Rumple), becoming the new Dark One again, with all the accumulated powers of dark-oneness. But unlike Rumple, who seems to have learned to symbiotically contain the darkness without going all out rampaging evil, Emma’s Evil Ash can’t do that, and becomes something truly monstrous.