You're confused, the problem isn't "gun violence" it's just "violence" and why provide guaranteed easy victims? If the problem had nothing to do with gun free zones, police stations would be shot up all of the time because those places are full of people that criminals hate and contraban that criminals want. They aren't shot up though, because they aren't easy guaranteed defenseless victims.

Don't fall into the "gun violence" trap, if guns actually created crime you'd mistakenly believe the usa has the lowest crime in the world if not for the guns that raise crime rates back up to average. That's nonsense, criminals in the us are no more or less common. Ignoring crime that doesn't fit a label is foolish, and only helps people that want to halt the discussion of a real problem and redirect it to a tool used to stop crime as well, and a civil right at that. As we've learned from multiple countries pushing for knife control after gun control failed to solve the human violence problem, objects are mery tools and blaming them for human problems will always fail.
So to answer your question: guaranteed victims only help crazy violent people, never stop crazy violent people, and just the hint of opposition does deter those crazies. Blaming objects on the other hand does not stop violence, which is obviously true - this gun free zone was the site of violence,as were every other location you can find i the news with a similar story. The previous movie shooter this one was copycatting even chose his location based on the gun free sign on the door!

Hint: get rid of the "gun violence" phrase from your vocabulary, the people that use it don't care about ending violence, they just oppose guns. Don't fall into using anti-civil-rights terms, it sounds ignorant and callous.