Wow, that's a messed up story. I guess the thing about having guns around to prevent shootings is that if a person is determined to kill another person, they will be able to do it if they aren't stupid. However, this situation isn't restricted to firearms, as any additional power given to a person could allow a homicide without much ability to prevent it.
Ever drive on a street without a barrier in the middle separating the lanes? Ever buy poison? Ever have a knife in your hand while somebody had their back to you? Ever have a baseball bat in while somebody nearby didn't have catcher's gear on? Obviously the examples are numerous.
In this tragic situation, a person took advantage of the shooting range setting, and was allowed to fire a single round at point blank range at somebody facing away from her. Trying to stop a crime like that would be nigh on impossible. But trying to stop a suicidal driver from veering into oncoming traffic at a random time would also be nigh on impossible. An intelligent motivated attack cannot be foiled IMO.