@AngleBoi7 it doesn't matter if anyone else knows. Mu kills folk discreetly already, and ruining her reputation is easy enough.
If you know someone is an attempted murderer, do you just shrug and say "I'm above violence, trying to harm people is wrong and uncivilised", and let that person go and likely go on to kill or abuse other people?
Just like that idiot son of 3 legged crow is preaching?
Or do you take action to stop someone from using their power to harm other people in future?
This is EXACTLY an example of what was being taught by the teacher. And Mu and the girls are doing exactly what the kid is suggesting to do, passively do nothing.