Mass shootings are terrible events for those involved. The victims did not "deserve" their fate, but they did allow it to occur. For their failure to choose life, they are to blame. For their deaths, they are blameless. The perpetrator is always to blame, for that person pulled the trigger and shot the victims. We, as a society, cannot find all those who will choose to murder before they act, so we must be ready to defend ourselves if we must.
Gun control, even effective absolute bans (which have never occured), will not prevent mass slayings. The greatest modern example of what can be accomplished by dedicated people without firearms is shown by Rwanda. Eight thousand dead, per day, for 100 days. Done almost entirely with machetes.
Absolute, completely effective gun control might reduce the number of mass slayings, or perhaps reduce the casualties of each event, but weapon controls will not prevent all mass slayings. A dedicated, patient perpetrator will accumulate weapons (e.g. large knives) and practice with them. Against the modern citizens of the U.S., a person armed with two knives could kill everyone in a building unless the police (who have guns) stop them. This does not require an evil person of action-hero skill and luck, merely a person who practices with their weapons of choice and a desire to kill everyone.
The reason this can happen is simple. The victims allow it. Look at the footage from security cameras at Columbine High School, read the reports coming out about the Virginia Tech shootings, read the testimony from the (few) survivors of Rwandan massacres, or the reports regarding the Australian shooting spree in Port Royal. Note the elements in common: victims attempted to hide, pray, or run. Some covered their head with their arms. Others played dead to escape further attack. All the victims waited for someone else to save them.
None of the victims died struggling with the attacker. None were hurt while flanking an attacker to achieve tactical surprise (and failing). None died while taking offensive action to save their life, or the lives of others. In cases where at least one "victim" fought back, the results are far different.
As a recent, and illuminating, example, take the shooting spree that occurred in 2007 in the Trolley Square mall in Salt Lake City. Young gunman, with shotgun and pistol (never used?) opens fire in the mall at night, killing 5 people. An off-duty policeman from Ogden draws his concealed carry pistol, and engages the shooter in a firefight. The Ogden policeman does not kill the shooter. But, by attacking the shooter, he forces the shooter to pay attention only to him. Thus, the large number of people in the mall still alive run the hell away from the continuing firefight. After ~3-5 minutes from the first shots, the tactical response teams of the Salt Lake Police arrive and kill the shooter. The Ogden office is unhurt.
Many in the pro-gun community use the event to promote the arming of the populace, and the issuance of concealed weapons permits. Many in the anti-gun community use the shooting as a whole to promote further restrictions on gun sales and ownership. Both are correct to a small degree, but mostly wrong. The important fact of the Trolley Square incident is that citizens unwilling to simply lie down and die are much more likely to live.
What has been lost in this country (and other western nations) is the idea that each and every citizen is responsible for their life. The police, military, or other citizens have no duty or requirement to protect your life and safety. You, and you alone, are responsible for your safety, in all aspects of life.
When citizens embrace this responsibility (as the fundamental duty of each citizen), the populace will not behave as unthinking sheep, they will not lie down and die when attacked, and they will not suffer a government that harms them. Freedom of speech, religion, expression, and so forth exist to safeguard the ability of citizens to fulfil their responsibility (duty) to maintain their own safety.
There are many effects of convincing the citizens that the government (at any level) is not ultimately responsible for their safety. Here, only three are detailed but the reader is invited to ponder how responsible citizens would act to determine further effects. First, there will not be a sense that ordinary citizens cannot be trusted with dangerous weapons (firearms, cars, saws, etc.); citizens don't want to die, they recognize that they are responsible for their safety and thus, citizens are responsible for all the outcomes of their actions. Stupidly injuring or killing another citizen without cause is grounds for prison, and they will be prosecuted for such.
Second, citizens who believe themselves repsonsible for their own saftey get used to assessing, continually and accurately, their level of acceptable risk and the risk currently occuring from their activities. Thus, there is no need for helmet laws, seat belt laws, or other protect-the-people-from-themselves foolishness. In a nutshell, responsible citizens realize that "gross stupidity deserves death" (by accident, not murder).
Third, violent crime will become significantly more risky. Responsible citizens will play along with a mugger only to achieve tactical superiority. Successful criminals will require such overwhelming force that no citizen could successfully resist, and it is hard to hide three guys with machine guns in an alley. Crimes without violence will be far less dangerous, for cars do not try to kill the criminal; a robbery is not violent until the weapon is drawn, at which point the victim rightfully believes someone will die, right here, right now. Responsible citizens will endeavour to make sure it is the criminal who dies instead of them.
Note that only citizens are responsible for their own safety; children are not citizens, and their parents are responsible for their safety and their training to be responsible citizens. Children need selective pressure (that is, ideally, non-lethal) to learn that their actions have consequences and prices that they must pay. "Protecting the children" is the worst reason to make a law, or impose a regulation.
To come back to the original start of this little essay. Assume a populace of responsible citizens is attacked by an insane, trained, equipped, and dedicated shooter. The shooter brings multiple semi-automatic, legal firearms with reloads. The weapons used are irrelevant, but assume legal semi-auto weapons for marketing impact. Assume the shooter finds a large collection of "victims" in a single building, and he locks the doors to prevent flight.
Unlike the modern public, when the shooter opens fire on our hypothetical public of responsible citizens, they will seek hard cover to buy time to figure out where the shooter is, and how many there are. At that point, the citizens will not pray for deliverance or beg for mercy. They will choose a route to the attacker that minimizes exposure, and possibly even coordinate with another to present multiple targets from multiple sides. The citizens flank the shooter (who is busy trying to kill people behind tables, or moving rapidly) and launch a close-quarters assault to overwhelm the attacker. Upon beating him to death, they call the cops.
This assumes the citizens cannot wait for a reloading interval (which is 1-3 seconds at most) for their rush, when they might be able to get the attacker with no casualties. Not all citizens will live, and perhaps the first rush (or three) fail and the attacker continues. Regardless of the outcome of the citizens counter-attacks, they will have died attempting to stop the threat, not cowering waiting for someone else to save them.
Of course, if the citizens are themselves armed, the scenario becomes much different - an attacker opens fire and then must seek cover from return fire of the citizens. While the firefight ensues, unarmed citizens can escape or seek better hard cover; armed citizens can flank the attacker to end the firefight.
Since the citizens are responsible (which is a state of mind, not so much training), they will not go hunting a shooter in a mall if they are not presented with the threat. Thus, the citizens do not start shooting each other - they return fire, they do not initiate a firefight.
It should be noted that people do not need extensive training to become responsible citizens. First and foremost, they need to choose to become responsible for their safety and actions. All else flows readily from this simple to make, hard to keep, choice. Many may choose to train themselves in the safe use of weapons, particularly if they choose to carry one should they ever need to use it. Many others will choose not to carry weapons, and rely on their bodies and found implements (pencils can be deadly!) should they need them.
Everyone who chooses to become responsible for themselves will, of necessity, practice risk assessment and mitigation so they don't die. As all of life presents constant risk at many levels, everyone has plenty of chances to practice where the consequences are not lethal. We currently call this experience.
Become responsible for your own safety and life. Everyone builds the life they choose, so start making choices that will better the odds of your survival and success.