Does anyone know why his mother didn't have her guns better secured from her mentally unstable son? I'm thinking this is step number 1 in what could have been done to have kept this from happening.
Sadly, I really don't think there is anything that can reasonably be done if we want to continue to have a free and open society.
By the way, this school did have its doors locked and the killer forced his way in. Even if a police officer had been stationed there, the killer would have probably shot him before the officer could have done anything, especially given the type of weapon the killer was using.
To make certain things illegal doesn't matter to certain elements of society. Drugs such as heroin, cocaine, etc. are illegal, yet plenty of people still manage to be able to find them. As much as it pains me to say this, the same thing would happen assault-type weapons.
Controls only work on the good people.

Ask the Sandy Hook parents if they'd be willing for their child to be hurt vs. dead.I think there is a lot of truth in your post, especially the bolded.
I agree, those intent on committing violent crimes will find a way.
There was an article shortly after the tragedy in CT, about a man that stabbed 22 children in China. There were no deaths but there were many hurt.
http://www.usatoday.com/story/news/world/2012/12/14/china-school-stabbings/1770395/
It's false to say that restricting access doesn't help.
In 1927, the worst school killing in US History took place. The weapon wasn't guns, it was dynamite. After the Bath killings, dynamite became severely regulated and there hasn't been a mass killing using dynamite in over 80 years since.
On April 19, 1995 Tim McViegh used fertilizer to make explosives that were used to kill 168 people. In the wake of that attack, rules were put into place limiting the amount of nitrate based fertilizer you can buy and the purchases that are allowed are tracked. There hasn't been a mass killing using nitrate based fertilizer bombs since.
In 1996 in Sydney Australia, 38 tourists were killed by a gunman with a semiautomatic rifle. Following that attack, guns were heavily restricted. There has not been a mass killing in Australia since then and the overall murder rate is down by more than half.
We're clearly never going to stop all violence and we're also never going to get rid of all guns. That doesn't mean we shouldn't do anything though. There clearly are measures that we can take to curb the worst of the problems while at the same time not overly restricting people's freedom and right to protect themselves.
My belief is it's because how we are raising children these days. From their earliest days they are taught that everyone is a winner, everyone gets a trophy, everyone gets to play whether you are good at it or not, everyone passes and moves on to the next grade, there are no losers, etc. etc. We are coddling our children. Life is not all sunshine and roses. They are not prepared to be disappointed in life, and have not been taught any coping skills. So they end up a young adult, out in the real world and it slaps them in the face. Not everyone gets a job, or a raise, or the award. Not everyone likes you, your boy/girlfriend dumps you. Getting something/somewhere in life takes hard work, and even then it might not be attained. They are now too old to stomp their feet and throw themselves on the floor in a two year old's version of a temper tantrum. Instead, this is what we end up with.I would ask too, why the escalation over the past few years? What's different with these young males that pushes them to commit these horrific crimes?
Guns may have changed but guns have been around since the country's inception. We did not hear about a lot of these outbursts until 15-20 years ago. Gun laws have become more restrictive yet we have more of these incidents. As people have posted, criminals do not obey the law. Anyone who wants to find a gun by illegal channels can and will. I'm not sure that further restriction is not a feel good measure. It's a frightening dilemma because in the end no matter what laws or restrictions, no matter how locked down and safe we think we are, someone determined to cause harm, will.
More school security. Mandatory ID's to be carried and checked by ALL students/staff. Secured doors leading to/from school.
As my father says, "If you outlaw guns, only outlaws will have guns".
I am all for stricter gun control though.
If you ask DH, he would say that if you are caught with semi-automatic & automatic weapons it should be mandatory 5 years in prison.
My solution would work, but nobody would like it. And all that has to happen is that I have to be made dictator.

My belief is it's because how we are raising children these days. From their earliest days they are taught that everyone is a winner, everyone gets a trophy, everyone gets to play whether you are good at it or not, everyone passes and moves on to the next grade, there are no losers, etc. etc. We are coddling our children. Life is not all sunshine and roses. They are not prepared to be disappointed in life, and have not been taught any coping skills. So they end up a young adult, out in the real world and it slaps them in the face. Not everyone gets a job, or a raise, or the award. Not everyone likes you, your boy/girlfriend dumps you. Getting something/somewhere in life takes hard work, and even then it might not be attained. They are now too old to stomp their feet and throw themselves on the floor in a two year old's version of a temper tantrum. Instead, this is what we end up with.