Oh. I was wondering if the root of all this is jealousy from a love polygon.
Like this:
Mei (♀) -> Taku (♂) -> Renjou (♀) <--> †Sahara (♂)
If we suppose that:
Chiaki (♀) -> Renjou (♀)
Then,
Chiaki likely killed Sahara because Renjou was in love with him.
Taku is the next victim because he likes Renjou.
Chiaki is eliminating her rivals for Renjou.
But how does Michi fit into the equation though?
She and Renjou did not know each other.
Her brother Kenta is also a wild card for now.