If you kill a person (within certain parameters), you become as evil as the person you killed.
As Regina said, Gold stole Cora’s life. It’s possible that the price for using that magic is that, in some way, you become as evil as the person whose life was taken. Maybe Rumple -is- Cora now. *shudder*
But that would explain a turn toward the darker part of himself.
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Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing