6'3" kid? Dressed like a full blown SWAT member? Probably holding weapons in both hands and firing.
I think the Avengers were busy with something else.
A lot of the people he shot were shot in the back as they tried to flee to safety. I don't blame untrained civilians for reacting in the most normal way for people to react but a concerted counterattack would probably have overwhelmed him. The problem is that there was no time to organize one and if you aren't already trained and thinking that way, you aren't likely to discover it in the first minute or so of an attack. A person with a pistol firing could also have drawn his attention (and fire) while others acted but it is all moot at this point.