Maybe someone caused Mako to kill herself by inflicting mental torment upon her and this is how Mizuki is trying to make the person responsible seem like the guilty party. She's pretending to be the murderer while deliberately leaving holes in the story so it might seem suspicious and would cause an investigation. She might have even left some fake evidence behind. I guess only time will tell.
This looks like an unreliable narrative style. All the flashbacks are only shown from Mizuki's perspective, so they are not necessarily true. Could be all lies.