Hmm…maybe Mal corrupted the Shadow but could be either way. I do not remember the shadow telling Mal to do anything but it did take Rumple away, so did do as Mal believed. Perhaps it did tempt him offscreen, but then the way it said mal broke a rule makes me think it is a supernatural creature that always existed, a embodiment of an idea like the greek myth gods. The sandman, or a guardian of dreams.
So there is that Andersen story about a shadow that takes over, so could be what happened.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass