In Chicago we have a ton of handgun-related homicides, and handguns have been illegal to possess for about 15 years or so (save for about 5-6 months early last year when the ban was overturned briefly).
I honestly don't think more gun control would stop things like this from happening. Mass producing firearms is a huge industry and not all of the weapons are going to go to law enforcement/military. Unless the production of firearms is limited (which would meet much more contention than restricting possession of firearms, I imagine) you will see firearms available for sale on the street market.
Simple precautions like metal detectors and properly trained security personnel like Godsmak suggested go a long way to deter/prevent tragedies like this from occurring.
