Yes, I think Hades is probably the one who created the Dark Curse. When you boil it down to its basics, the Dark Curse takes away Happy Endings, the very thing that Emma restores. I think Hades and Emma are being set up to be opposites of each other (the enemy of life, the savior or life;) It would make sense for him to have created the Dark Curse that she was destined to break.
So it wasn’t Merlin? I forget didn’t someone re-con say a Soccer create it or again am i just hearing things…My ONCE canon (little though their may be) Is kinda lose even with the Retcons.
They better explain how Hades can relate to the Dark One if not by a connection (Though that may be possible) at least through who’s the oldest and more powerful of the two.