Gun or not, the criminally-insane will always find ways to kill. You can make about 100 different types of bombs using household chemicals, all with far more lethal and discreet potential than a gun. Maybe we should start burning all cleaning agents.
Take a hard look at history, namely Prohibition. What happens when you restrict access to a resource in demand? People find new, illegal, and potentially more dangerous ways to sell it.
Let's disperse with the idealism that regulating/destroying guns will somehow create world peace. It will create new opportunities for criminal-entrepreneurs. Black markets. Increased crime. More guns in the hands of criminals, and less in the hands of honest people (I don't particularly care about how many "honest" people have guns, but I do care about the consequences of over-regulation).
People with mental health issues usuaaly won't have the techincal know how/ skills to build a bomb using toilet cleaners. I come from India, where there are groups that make low tech bombs. Not individuals. Often times they injure themselves than kill a lot of people. The ones that kill 100's often require well regulated chemicals.
Mental health does not always negate mental power. Sociopaths and Psychopaths (a very similar classification) have perfectly normal brain function, only (to some degree) violence does not bother them -- some even enjoy it. Not all socio/psychopaths are killers though, in fact some are very good people (it has been theorized that a few US Presidents were sociopaths). Often times sociopaths can be very intelligent people, especially with regards to social hacking (they can be very charming on the outside, and very manipulative). (Source: my parents are psychiatrists who deal with the criminally insane, among many other types of mental cases).
And yet, countries with strict gun control tend to have far less deaths caused by guns.
Nobody is saying that gun control will mean that nobody will ever get shot and killed again, but the harder you make it for people to act on their murderous feelings, the less likely they are to do so.
It is objectively more difficult and time-consuming to build a bomb than it is to point a gun and somebody and shoot them.
Lots of people have feelings of rage at some point in their lives. Most people don't act on those feelings in a destructive way.
Even though most of these school shootings in the US don't seem to be random - there usually turns out to be at least some form of pre-meditation as far as I can tell - the fact that it is so easy to act on your feelings of rage / destruction cannot do anything but lead people who have those feelings down that path.
Murders in the U.S. have decreased steadily since 2006, dropping from ~15k to ~12k. Firearms murders, which made up 67 percent of all murders in the U.S. in 2010 have followed this trend, decreasing by 14 percent.
California, a state with one of the most strict gun laws had the most gun murders in 2010. 69 percent of all murders that year, according to FBI data.
The point is not that gun control reduces the number of murders: it's that making it harder to get hold of a gun makes it harder to kill somebody.
Anybody who wants to kill somebody badly enough will do it with or without gun control: you'll still be able to get hold of a gun regardless. What gun control will do, however, is make it much more unlikely that somebody will go out and act on their murderous feelings.
To the man with a hammer, everything looks like a nail. To a man with a gun, it looks like a valid solution to far more problems than it actually is.
And yet here we are: a man just shot up a school full of kids.
A gun is an effective way to kill somebody - regardless of what happens after the fact. Most of the time in these kinds of massacres, the perpetrator doesn't care at all what happens after the fact - and I'm willing to bet that in a lot of gun murder cases, the perpetrator was not thinking about the consequences / ramifications at the time they pulled the trigger.