Why was Yoshimi killed?
I wondered the same thing. Is it possible that perhaps she never graduated elementary? Saki's mother was very excited when Saki's spirit came to her finally...throwing herself into her, it could be from feeling another failure for her last chance as a mother. Or perhaps she moved up quickly but then became stuck in a rut like Shun.. Maybe she was a student much like their 5th partner who didn't last long. Her inability to gain any traction on her powers may have been looked down on..maybe it depressed her sister..and had the opportunity to bring rise to a demon. perhaps the child seemed too problematic and posed a threat to become the next demon