There is a reason police officers are trained to shoot only when they see a weapon being pulled on them. Apparently, you feel police don't kill enough citizens for you taste.
That's pretty sick, in my humble opinion.
The world we live in today includes weapons other than guns...like explosives strapped on one’s body. She was also in a high security area.
Given the recent events at DC (people setting themselves on fire, etc) and given the high profile shootings of the recent past...I have no doubt the security there is on edge.
If she had blown herself and a bunch of bystanders up...people would be screaming “Why didn’t the police stop her??? They had the chance!”
We have the luxury of 20/20 hindsight. That was not the case as events unfolded real-time.