If she got through? That was an inevitability if she was not shot. She was literally climbing through the window with nothing else to stop her at that point.
Yes, IF they broke the door down, it would have been worse. More people would have been shot. They didn’t break though the door though because the barricade held. The window did not.
1 person was going to actually get through. The cop decided he could not let that happen, and shooting her stopped the mob from doing something worse. He was effective in his 1 gunshot.