I am not trying to justify their actions, but it occurs to me that Regina and Rumple both turned to the dark side because they were tired of being victimized, and because they lost someone that they love.
Regina loved her father, but even she pointed out that he did nothing to protect her from her mother. Her mother killed the love of her life, and then forced her into a loveless marriage with a man who essentailly kept her trapped. It is easy to see why Regina would want to take matters into her own hands.
Rumple was considered a coward by pretty much everybody. His wife left him, and it is implied that she cheated on him. Then these guys come along and try to take away his son, who is the only person who seems to see any worth in him. These same guys then procede to humiliate him in front of his son in a scene that I may never be able to watch again (where they make him kiss the boot). Rumples fear made him let Bael go through the portal alone, and it is possible that Rumple's goodness, or a great deal of it, went with him.
Just as Rumple's dark side cost him Bael, and may cost him Belle, Regina's dark side cost her her father, and may cost her Henry, Yes, it is clear that what Rumple said is true. "All magic comes with a price", and this seems to be particularly true for dark magic.
So the question remains, will these two continue down the dark path until it costs them everything (maybe even their lives), or will one or both of them attempt to make amends for the harm they have done, and heal the holes in their own hearts? The only thing that seems to be certain is that neither one of them is one dimensional, and it is probably best to not assume you know what they will do next.