The presence of another weapon only increases the likelihood of more deaths in these situations. The 22 year old could have easily became one of the targets. Based upon the stories - he was out-gunned. It isn't too difficult to imagine the consequences if the police or security came upon a 'second' shooter.
It doesn't make me comfortable to know that there are people carrying guns in the Mall, or any mall. Arguably there is not much difference between the 'bad' mall shooter and the 'good' mall (potential) shooter. The existence of a 'permit' doesn't prevent wrongful firearm use. The requirements for a concealed weapons permit isn't that stringent.
The apparent mental illness that triggers these mass attacks is not obvious. According to a July, 2012 Psychology Today article Mass Murders Are On The Rise "[s]evere mental illness in and of itself was not a predictor." And "the mass murderer is typically a white male, a loner, has a college degree or some college, from a relatively stable background and from an upper-middle to middle class family."
[See this story published after this post: Sandy Hook: Police say shooter forced his way into school. The shooter falls well within the profile above - "the mass murderer is typically a white male, a loner, has a college degree or some college, from a relatively stable background and from an upper-middle to middle class family."]
Thus given what is known about the mass shooters, it is not likely that meaningful legislation can be devised to prevent mass shootings. And having that discussion post a mass shooting is irrational - because the causal connection between the shooter and the event is undetectable and, therefore, unpreventable.
That is not to say that efforts shouldn't be made to reduce the amount of weapons available to the public under the guise of constitutional rights. One doesn't need to have a mass shooting to be a catalyst for discussion about the availability of weapons in our society. Adults, and children too, are being killed daily by guns. That availability is difficult, if not impossible, to justify in a civilized society.
The vast majority of people go about their daily lives without the need for a gun. But I can see the desire of many to fire those types of weapons and even to collect them. These people are not the shooters that we need to worry about. But, surely there are legal means to reduce the availability of firearms while accommodating those who find it entertaining to fire or collect firearms?
Maybe a place to start is to have that rational discussion about preventing the death of children from firearms. The statistics are alarming. According to the Center for Disease Control "in the latest reported year, 153 Canadian children were killed by guns while France had only 109 children killed. In Japan, the number was zero. The United States had 5,285 childhood gun deaths that same year." [Thousands of children are killed by guns in the U.S]. See too a 2012 Chicagoist article More Illinois Children Die From Firearms Than From Car Crashes.
Surely failure to protect children from firearm deaths cannot be justified by protecting constitutional rights of adults?