Don't you think it's an interesting "coincidence" that these things have a tendency to occur in the USA and in Finland, two of the countries in the world with the most liberal gun laws?
The point is that the kind of people responsible for such shootings are not criminals per se. They're disillusioned young people who're fed up with society. Probably very introvert too. They probably wouldn't even know how to obtain a gun illegally. If Finland had had the same gun laws as here in Sweden for example, this shooting and the one that took place less than a year ago in another Finnish city would never have taken place I'm pretty sure.
Of course you could argue that if he hadn't used a gun, he could have used a knife or whatever, but then he surely wouldn't have been able to kill 10 people.