I mean, nothing came as a big surprise here, but why on earth would they let Reva live?
They are literally hunting force-sensitive beings all around the universe just because they might turn against the empire. She did turn against it, and although we cannot say how powerful she really is she has had training, and will use it to help bring the empire down. Why would you leave someone alive who has intimate knowledge of many secret things, and actually tried to kill both persons who were present there?