 Well, securing the building from the inside does not prevent an assassin from using the outside to get on the roof. The question was, should the building have been secured? If the building was secured, you didn't have to put agents' lives at risk on the roof of that sloped building. Apparently, it wasn't that big a risk for the assassin. You could have had sniper eyes on the roof. You could have secured the perimeter of the building.