I think people's hatred towards Stona is misplaced. The sacrificial rituals and, in turn, Rei's unfortunate death were a result of the twisted society they lived in, which Stona had no control over.
As repeatedly demonstrated in the chapter, their society had an almost insatiable thirst for blood. This is evident from the "random, blind killings" that occurred before the introduction of the ritual, the ritual itself, and the public's demands to redo the ritual after Fone's rescue attempt.
While the ritualistic killings may have stopped after the uprising, the story suggests that this would only be temporary. The rebels likely opposed the sacrifices because they shared in Fone's belief that the ritual was pointless -- no god was waiting for the chosen's blood. But after their bloody rebellion, they hail Fone as a "new god." Once strife inevitably appears, with the people's belief in a higher being now reignited, they would quickly resort to human sacrifice once again. I believe this is what is meant by the final exchange,
"But nothing had changed, had it?"
"No."
In short, regardless of what form it takes, their society would continue to demand blood. As Stona recounts, a priest's previous attempt to stop the ritual resulted in his death by burning. As she puts it, "the people had changed... without blood, they could not be at ease." Any attempt she could have made to end the ritual would surely have failed. Had she tried, she too would have been killed and subsequently replaced. Nothing would change.
However, the futility of Stona's position doesn't stop her from at least trying to do good. She goes against ritual customs by letting Rei wear her beloved sandals, attempts to postpone the next ritual to 16 years rather than 8, and offers up her own life for the ritual to prevent another innocent life from being taken. Even the priesthood did what it could by celebrating the chosen's family and lover and using "[their] medicines to kill painlessly."
The fact that the mangaka includes these details suggests that he doesn't want readers to blame Stona/Kouko. Rather, she is a victim of this twisted society, much like Fone and Rei. Calling her a "bitch that deserves to die" is ignorant of what the story is trying to convey.