Akari’s instant infatuation towards Menou is explained in Episode 6, when we learn herself from the future can hijack her body and always “implants love for Menou” before she “leaves”.
Flare herself is quite the Well-Intentioned Extremist but the thought of how many people’s blood is on her hands is rather unsettling for herself and them.
Menou is a young girl and already she has been forced by fate to kill so many for the sake of the world. Some cannot imagine killing one person for self-defense let alone multiple for a good cause.
