To the confessed murderer, Qtalda and her merry band of collusionists, I say "payback's a bitch, bitch! Hope YOU like being 'erased.' It's for the good of the country, after all. We can't have the Conclave go 'rogue' and start slaughtering citizens without trial now can we?"
Hm. It's that cut and dry? Suspira's really not that much better than the Conclave to take the law into her own hands, you know...
Yes, it is. Suspira is clearly acting in self-defense. That is NOT taking the law into your own hands, anymore then shooting an armed robber who breaks into your house. The Conclave (especially Qtalda) has not only confessed to murdering her parents, but have graphically demonstrated their intent and ABILITY to murder her, and upon failing to do so, have stated their unwavering intention to continue attempting to do so until they succeed, regardless of how many others get killed in the process. I should point out how Qtalda told the rest of her accomplices to search the woods for witnesses and "erase them."
Suspira, on the other hand, has given Qtalda every opportunity to not only confess, but to turn herself in. Heck, she even let her doll be "captured" and put up no resistance whatsoever. In fact, Suspira did not cast a single spell on any of the Conclave members, nor did she so much as lift a finger in their general direction. She did express outrage at Qtalda's blase admission that her parents died "for the good of [the country]," and the fact that she held the entire conclave responsible for their murder, but did not mention a single threat at any of them. Had they given her a fair trial instead of attempting to summarily execute her, they'd all still be alive.
At worst, Suspira is guilty of assaulting her fellow students, but as we saw with their behavior at Kin's funeral, when she was eating dinner with Crest (where they assaulted her), and as they were heading to their dorms (thinking they were unsupervised talking to each other, planning further mischief directed at Suspira), she may well be justified in her attack on them. Halcyon also pointed out that this action is not deserving of death.
I do agree that we have to wonder what Suspira will do now. She is still quite angry, and bitter, and I don't blame her.
As to the three "Mary," I had a rather disturbing thought. Maybe they will be Poly and Bernadette's opponents. It's a pretty sweet deal for Mary and that country.
There really is no "downside" for her. As long as she's in the arena cells, the phalanx can't touch her. If she kills her opponents, that just means more marks (which means she gets to stay in the cells longer, safe from the phalanx), and catered meals. (Yum)
As for the country, heck, they get rid of their strongest and most dangerous criminals who simply can not abide the "no violence" rule, because only those with a great many marks would choose the arena.