if this was a shooting at a mosque or an atheism convention and people were picking that time to say "it's cause you serve the wrong god" or "if you were a ____ this never would have happened" it would still be wrong.
believers, non believers, whatever you are, don't pick times of tragedy to beat somebody up over their beliefs or lack of. they are going through enough already. if that person wants to talk or has questions about your faith that is ok but be nice. yelling "YOU'RE GOING TO HELL" or "YOUR PRAYERS DO NOTHING AND THERE IS NO GOD YOU MORON" do nothing but take what little this person has to comfort them away.