In contemplating the tragic circumstances in Roseburg, San Bernardino, Colorado Springs and elsewhere - where terrorists, loons and terrorist loons went on homicidal rampagesone is faced with multiple choices in how to react.
You can tear at your hair and gather sack cloth and ashes for a rousing session of emotionalism with a strong theme of "there ought to be a law!" and "we need more gun control!" This lacks a certain amount of rationality, maturity or likelihood of actually accomplishing anything in terms of preventing shootings, mass shootings or the heat death of the universe. Murder is already very, VERY illegal (as is assault with a deadly weapon) and yet another law seems dreadfully unlikely to deter bad actors from acting badly.
You can campaign to address parts of the problem. While overall violent crime is down, mass murder is never a good thing and worthy of extra effort. This approach is somewhat more productive, if a bit long term and speculative. In this model you can work to improve immigrant screening, better fund domestic intelligence efforts, improve the availability of mental health care (and reduce the practicaly and perceived stigma of seeking out such care), seek to limit over-criminalization and work to isolate demonstrably violent bad actors - all this is to the good.
Despite the generally worthwhile nature of this approach (and I'd argue that if naught else, you score moral brownie points for such efforts), such long-term efforts fail to address the immediate "crap happening right here, right now" range of situations. These longer term efforts fail to address "What is the right action should it all go south right there in front of me?"
"Can't happen to me..." really kind of sucks as a plan. It may be unlikely, but the fates may make the choice to crap in YOUR hat one fine day and assuming that you are magically immune doesn't really prepare you for a better outcome than dying messily. Thinking it through and talking it over with folks you respect might at least allow for the chance of a substantially improved endgame.
Sometimes the very best option left on the table is to decide to die falling forward - the exits are blocked, the bad actors have numbers and logistics on their side, and life just generally sucks and appears to be a short term phenomena. A "sucks to be you" kind of day, if you will. The best option you have is to take down as many hostiles as you can on your way to the funeral parlor - if nothing else, every hostile you take down is one that is unlikely to ever get all anti-social with someone else in the future.
The key to doing this successfully is having made the decision, far in the past, just what the bar is for your personal final charge to glory - how bad does it all have to be before honor outweighs survival?
Give it some thought, and then some prayer you never find yourself in a situation so fouled up that it becomes a practical consideration.