Given that early Rumple wanted to use the Dark One’s power for good, and we saw how quickly that changed, there’s no question that the power has corrupted his purer instincts (which weren’t the purest to begin with–he did desert from his military service for sympathetic but nonetheless dishonorable reasons, and he did choose to burn the duke’s castle and kill Zoso all before he became the Dark One).
I think he used his dealmaking to help control not only the cost of his magic to himself but also his darker impulses. As long as he had Baelfire to keep him human, it was effective. Without his son, even with Belle left to love, there’s not been enough to keep it in check.
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