They did "drop him." None of this is immediate, and only the ignorant think you can -prevent- someone with evil intent from acting. All you can do is be more or less mentally prepared, trained, and equipped to respond in the most effective manner.

Trying to locate a reasonably competent shooter at night, in a built up area, in the middle of a crowd scene gone to panic, is a nightmare scenario. Yet the police did it while protecting the innocent. Once they had him located they engaged him in a firefight and took some additional losses. But by engaging him it meant he could no longer shoot at will but had to take cover and deal with incoming fire. That saved more lives than it cost. Counter-ambush 101. They were able to maneuver to bring fire superiority on him and got him pinned down. At that point the decision to use the robot was made rather than send officers in to dig him out and potentially be harmed. Once deadly force is justified the means used is largely irrelevant.