There is evil in the world. There has always been evil in the world. There will always be evil in the world. There are also mentally unstable people out there. No law will keep people from being evil. No law will keep people from mental problems. Besides evil and mental issues, there is the drug trade and other criminal enterprise, which may not be evil, but certainly illegal.
What can we do, then? Well, for one, we need to work on our ways of dealing with mental illness. Currently, we stigmatize it, keeping people from seeking help. Then, if help is sought, people look for the simple and easy cure--they take drugs until the symptoms subside (if even that long), and discontinue treatment. Those beyond treatment get committed, but the facilities are both temporary and ineffective, so they end up letting people out without proper care.
Our prison system seems to have trouble deciding whether it is designed to punish or rehabilitate. If it is the former, we offer too many amenities--cable, education, recreation...often better than the working poor. If it is the latter, the recidivism rate indicates we aren't doing it right. Also, if we can trust someone with freedom, why are they still stripped of their rights?
Our war on drugs has been horribly ineffective, and drug peddlers are a major source of violence. We should rethink the whole thing, in my opinion. If we legalized and regulated the drug trade, the criminal drug traffickers would find their business worth a lot less violence.
As for the mass killings, true evil finds a way. A bomb, poison gas, or even a truck could be used to kill a lot of people. Someone who wants to kill will find a way. The best way to stop mass killings is to equip the public with the tools to stop killers. Train high-risk employees in spotting and stopping threats. Arm people in high-risk zones, rather than making them soft targets.
Improve our message to children. Reward them for doing good work, punish them for doing wrong, and stop making excuses. Yes, ADD is a real thing, but it is not an excuse, and not every kid has it. If Johnny acts out, punish him. Beyond that, teach kids to respect and safely handle firearms and how to mediate problems. Basically, make sure they have the tools to deal with their problems constructively.
Don't target the media or the firearms, target the criminals. Know that evil comes in many forms, be it serial killer, dictator, or mass murderer. And we need to be prepared for all of them. Taking away the tools to fight those evils will help no one.