^
We don't know what reasons Seo might have to want to kill Miho. We only know something bad happened involving them the previous school year, which might hold the explanation. Seo being in on Michiru's kidnapping plan does signal she was at the very least fine with her doing that to Miho.
Also, we don't know if the student council tried to kill Miho multiple times. They did with the poisoned drink plan, but that was after the kidnapping incident and I'm proposing they might have not been responsible for said incident. And Miho's first death was an accident with her getting a concussion from a bad fall. The poisoned drink plan was already set up without them having to rely on Michiru, and involving others without planning to also get rid of them would be very dangerous anyway since they could become testimonies or hold evidence. That's why they set up the lightning strike for Michiru together with asking her to paint the windows with ink for Seo's murder attempt.