The Dark Curse was a cyclical thing - a perpetual poisonous thorn in the side of whomever it stuck to like barnacles - because it was always recreating itself. A cyclical hate, and a constant cycle of madness - you feed the madness and it feeds on you - that kept chasing itself for centuries. All in your own head. And while the last Dark One, Rumpelstiltskin, had given in to that power he'd also resisted more than anyone knew.
Killian was not a person who could resist. He knew what he was, a selfish pirate who had been trying to be better, and this darkness felt more comfortable than it should have.
Teeth and jaws snapping within the confines of his mind, the sounds of all that cyclical hate and fury, it was actually the second one. His hand plunged into Garrett's chest, taking his heart out with it. That now enchanted heart, that was how it worked - once the heart was removed, it became a magical, enchanted thing.
You didn't hurt someone when you removed their heart. People could live without their hearts. You just...controlled them.
He held it in his hand, jaw clenched. "Are you so very certain I can fight?"
Crush it. That was from Rumpelstiltskin, sniggering in the distance. Like a clod of dirt, dearie.