Uk forces legally have to give a warning, which varies dependant upon the theatre of operations. We shoot to incapacitate the assailant - a mildly injured attacker (say shooting in the legs) can still cause untold damage, either with their own firearm or setting off suicide vest/bomb. I'd imagine the US forces will have similar guidance. Paint it any way you wish though, if she hadn't attacked a government building she would be alive today. We all know how gung-ho the federal services in America can be, I'm just surprised the death toll isn't higher. Such a "peaceful" protest has also led to the death of a police officer - it's time America had a serious, grown-up debate about their gun laws.