Board Thread:General Discussion/@comment-26412219-20150803161431/@comment-24674051-20150803174128

It is a complicated chain. That extends prior to the earliest point in the timeline that we have so far seen.

You could blame Rumple, Hook, Regina, Zelena, Cora, Snow, or even Zoso or the Seer. But I am going to guess that it was Merlin that created the dark curse (maybe we'll find out for sure), in which case, I blame him.