Do we know that's what the circumstances were?
Are we sure that the girl wasn't getting bullied and reached a breaking point?
Had a mental breakdown?
Has mental illness issues?
It does not matter because it is illegal to use deadly force against one who is not using deadly force. As far as I can tell, the girl in the pink only has her hands. She is not using deadly force so 15 is not allowed to use deadly force against her.
If she was bullied and reached a breaking point, she is not allowed to murder someone.
A person dead from one having a mental breakdown is just as dead.
A person dead from one having a mental issue is just as dead.
All valid points for matters that could and should have been dealt with BEFORE 15 grabbed a knife and tried to kill someone, BUT while she is in the act of trying to stab someone, THAT IS THE IMMEDIATE ISSUE AT HAND.