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).
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.
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.
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.
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.