This will certainly put an end to Regina's motivation to reform, but she's still committed umpteen atrocities over the course of decades and represented a clear-and-present danger to the main cast and everyone in Storybrook and everyone in the Magic Kingdom, and probably Wonderland and Oz and Narnia and the Federation and the Mushroom Planet and....
There's no going back for Snow because she has a conscience, but her actions are certainly defensible. If she had killed Cora herself, she would have been a soldier; tricking Regina into doing it makes it espionage. Things like that happened in Mission: Impossible
all the time.
That was certainly a great scene when Rumpy called Belle and reconciled with Bae. I wonder how far they will go with Rumpy's rehabilitation. Perhaps making Regina completely evil again will free him up to be the good guy.