>>713574858
I don't remember that happening in either version. In anime, Mion gets the schizo injection and does all the same shit that Shion did for no reason, killing Shion in the process(holy fuck anime wataakashi is bad).
In the manga, Satoko just injects one of them without knowing who is who, Shion goes schizo and loses her sense of self, does the "I never should've been born" shit from Meakashi and kills Mion thinking she is "Shion." Then later on she guns down Satoko and kills herself out of guilt for once again failing Satoshi's promise.