Only one side has decided that.
Let me explain. I have decided that I will never attack anyone. I am for peace.
If you attack me and cause me to be in fear of serious bodily injury or death, I will shoot you in the head to jeep you from ever attacking anyone else ever.
If you attack another under my care or in my vicinity I will shoot you in the head in order to keep you from attacking anyone else ever.
If you live peacably with me, yoy have absolutely nothing to fear from me.
I hope I never have to harm another, again. That was only reasonable non deadly force, but he never attacked me again. We were at peace.