This could only happen in the US.
Gangs and blackmarket thugs do not normally hit shopping centers of packaged people without a reason or target, and are normally not a threat when guns are involved, more like bombs.
Banning guns would have stoped idiots like this kid. Look at the UK, guns are banned, and apart from in London (Which is Gang Central, with kids as young as 10 being involved) there has not been 1 shooting in the last while where the target has not been suspected of being involved, or owed money to a gang.
Also - Bobbies are packing in the UK, with 60% of police cars being equiped with a firearm of one type, and 80% of police cars being equipt with a Taser style weapon. They do just not "pack" in public.
Plus - They say that 1 in 40 people you pass in a busy street in the London area is carrying, yet 9 in 10 of those, are licensed (Unmarked/Plain Clothed, SS, etc.)
EDIT:
I should just add, that in all definative terms, this "kid" was not a terrorist, and should not be treated as one, he was sick and was able to abuse what can easily be got in America because he was mentaly unstable, if you would just check the people who are buying guns, this should never have been sold to him, and you could have avoided 8 deaths. So don't ban them, but at least keep a record of who brought a gun and carry out "metal" or grudge checks against the person buying, if they have a record, don't sell to them.