Right now, someone is sitting thinking about how they can do what is necessary to kill as many people in as short amount of time as possible.
Don't think about why or when or how, but think about this someone with a thought process that drives them to this edge. Then think about someone else; this someone is going to be killed. It might even be you. Now those are facts.
Odds are the victims do not know each other or have never even seen each other. Doesn't this shout loud and clear we should do something?
But odds are what we will do will not prevent this person who has already begun the process of human destruction. Even though we have waited too long to bring about the end of this insanity, it does not justify that we sit by and agree to disagree on what to do. Aren't we smarter than that?