Sick Mind/theories

Kurayami killed her sister out of envy or from abandonment.
Very obvious that Kurayami feels envy or jealous, because her sister Yamitsuki get more attention than Kurayami herself not only from friends but even parents, but its not all even when Yamitsuki get a overdose from drinking drug she still have a care from everyone except younger sister, so feeling yourself abandon she get insane aka she get a Sick Mind so she decide kill her sister Yamitsuki and even Iratsuki who was Yamitsuki's friend maybe best friend, but in the end after bad deed Kurayami understand that she make a very bad thing so she feels regrets, too bad that is too late.