How to start a plot:

1. Someone teaches Samael the value of independence (in other words, accidentally eliminate her servant attitude to Sariel)
2. Make any evil semigod/god-class character find a way to unseal Samael's powers as she begs so.
3. Now free to rampage, the school must stop the maid from killing ten people to ultimately prevent her from gathering ten soul gems that will allow her to mass reap the entire school.