Rumpelstiltskin.
As the Blue Fairy traveled back, her mind kept jumping from scenario to scenario trying to figure out how Rumpelstiltskin got so ahead in his plans. Since she had met The Dark One all those centuries ago she knew that he was a force of evil that would be relentless in his pursuit of retrieving his son Baelfire.
His twisted sense of love had been the fuel to kept him hunting for a way to be reunited with his boy. He had no regard of the immeasurable pain he caused to others, either directly through his manipulative deals or indirectly through his cold, calculated plans. He really was a cold-blooded crocodile.
She knew that he already had a few of the pieces up on the board and that at some point in the future he would unleash his Dark Curse. Everyone would suffer, but it wouldn't matter to him. She had no idea how or when it come to pass, but now she discovered a new pawn in his game - The Queen.
This last meeting with Regina made her believe that queen was being coerced into working for The Dark One. What frightened Blue was that only now, after looking back over the last year and a half, did some of the pieces fall into place….