One of the parts of Cora’s personality is making people who formerly wronged her pay for their mistakes (in her mind) and show it. She turned the random dock guy into a fish because she didn’t care and needed to make sure that nobody knew they were in town. She lets others live so that they can watch as they fail. It’s probably where Regina gets a lot of her ideas on revenge: it’ll be more painful for them to not remember they love who they love and I get to watch them be separated. I get the last laugh. It’s very similar to Cora wanting them all to kneel because she never would again.
So my guess is that Cora and Regina don’t mind just tossing people aside and continuing on because it shows their power. They like to feel superior.
Keeper of Hook's Trenchcoat.