What if Dr. Doom!Dark One is the untethered Darkness taken corporeal form? Rumpel warned Belle that she doesn’t want to be around the Dark One when no one else is home, as his heart was going black, and he was dying. Also, in “The Miller’s Daughter,” when Rumpel was dying, the name on the dagger was fading away, which led to Cora changing her plan to trying to become the next Dark One. And, if she knew what would happen, maybe only Merlin can control the untethered Darkness, due to being the Sorcerer? Even she could have feared the untethered Darkness!
This would explain why Merlin accused Dr. Doom!Dark One of killing his lost love. We don’t necessarily know long Dr. Doom!Dark One has been around, after all. Rumpel was the Dark One for, what, 300 years?