Well, while I agree with getting over there and doing some damage to the asshole, I do think that shooting at 100 yards is a bad idea. They say when you're in a fight, you're only half as good as your best day at the range, and how many people are capable of 100 yard pistol shots on a nice, peaceful range day? So, you've got a miss, more than likely. You have ordnance flying through the window into a building full of innocent people. That alone is enough to make me want to close the distance. But wait! There's more!
Look around on teh interwebz and you will find several cases where shooters have worn body armor that will stop pistol rounds. If you fire at 100 yards, and the shooter has body armor, even if you hit him it won't do anything but alert him to your position. Once he knows you're there, you have no armor and one pistol + 100 yards' distance against a hostile with a medium-caliber, semi-auto assault rifle who is armored against your ammunition. You have, most likely, just cost yourself the engagement, if not your life. What happens to the people in the building if you are killed while running over there? They die anyway.
So, it seems to me that the best option is just to haul ass over to him and take him by surprise if possible. Take a head shot in case he has armor. More people may die before you get to him, but if you only take 15-20 seconds to run over, that number of people will be much lower than if you were killed after the opening shots at 100 yards.
Am I right?