I know I'll regret this, but here goes.
I don't see much of anything proposed yet that would work for anything more than a knee-jerk, feel good solution. I will say that I believe a large portion of the problem comes from the video games. Movies...maybe...somewhat, but the video games put the person in control of what is happening and they are getting more and more realistic everyday. These kids are getting a daily dose of violence and destruction fed to them in their entertainment.
I have actually witnessed in a neighbor's kid. It was obvious then and when I heard of the brutal assault this kid unleashed on those Newtown kids, I could envision my neighor's kid doing the same thing if he ever snapped. Thankfully, they did medicate my neighbor's kid...all through his teenage years (yeah LOS, I didn't believe in it either 10 years ago). He hated every minute of it too. You could see it, over time, start to calm him down. He was finally able to get off the medication about the time he graduated high school. He is a completely different person now. He has grown into a fine young man so far. Will he ever revert back? I have no idea, but I have witnessed that it worked so far.
I don't support gun control, but I personally don't need an AR-15 to protect my home either. My problem with proposing guards at school is that I don't think you can do it effectively. Most schools have several, separated buildings. You would have to have a guard in each building and then someone could easily ambush the kids between classes as they walk between buildings, or at recess in the lower grades (that's what happened here in Jonesboro, AR several years ago. My thought about an armed guard also is that just gives them a first target. Ambush the guard, take him out, then continue on....doesn't solve a thing.
So, if guards can't stop it and gun control can't stop it, what is the answer? I think they have to quit training kids to murder people. When the video games stop training them, they stop being so lethally accurate in how they carry out these acts. That won't solve the mental problems, but at least without the video games, they aren't being trained and encouraged to act out their rage.
I also believe that the lack of discipline in schools and at home is a part of all this. A kid is going to push whatever boundaries you set for them. We set very few boundaries these days. Breaking the rules means nothing to these kids. Over a period of time, as the boundaries of what they are capable of doing without any real disciplinary action get pushed farther and farther away, the actions get more and more violent. If you even raised your voice to a teacher when I was in school, your butt would get lit up enough that you would think twice about it next time.
I personally think that what is happening today is nothing more and nothing less than a culmination of the lack of discipline and lack of parenting that has been brought about in the last 20 years. Our generation has failed these kids, not by giving them too little, but by giving them too much. Some of the things we have given them are nothing more than outright tutorials for inflicting horror.