Generally speaking, I'm very, very strongly disinclined to judge magical girls for what they needed to do to survive. The conditions of the magical girl world are monstrous, and so they give rise to monstrous actions.
If we condemn everyone who failed to meet, say, Kyouko's level of standards, then we'll probably need to condemn something like half of all the magical girls currently alive.
I don't think it's particularly unlikely for Rionna, who at least mostly limits herself to "acceptable targets" and reprisals to attacks on her, to end up being on the upper half of the morality scale.
Akiko, who Sabrina has expressed distress for handling roughly, was forcing the University group into actual no-shit non-metaphorical slavery. The grief controller Rionna mentioned witched out "a few score", which is to say over forty, magical girls.
I think we can make some assumptions about Rionna's past at this point. She almost certainly wished to bring back the soul of her sister, or something along those lines. Her magic seems to have very few direct offensive applications beyond ripping people's souls out, meaning her wraiths are her only method of non-immediately lethal conflict. And she has some sort of drive that pushes her to hunt down and "punish" those that falsely claim to be grief controllers.
It's easy to see how someone in that situation would be forced to use her power to survive, and there's any number of tragic stories that can fit the facts as we know them. Nobody's the villain from their own point of view.
