I think the bottom line to this kind of thing is that there are psychos out there. This is not a USA only thing - mass shootings occur everywhere, it just seems to the USA has a system where guns are more accessible (Australia's gun laws changed dramatically after the Port Arthur massacre - see
link for details of the shooting - that even if I wanted a gun I have absolutely no clue how to go about getting one).
Also, American shootings on this scale make the news because most of the news outlets have a pretty US centric view - bombs going off in other countries probably don't the coverage in the US that a school shooting gets.
RE access to guns - personally I think it should be really hard to get one, even if the law says you can have one. Laws should be written in such a way that there should be a sizeable background check before you can buy one. If you are deemed to be mentally unstable, you should not be able to buy a gun - end of story.
I don't buy into the more guns = more gun crime argument either. I think this is a red herring. The kinds of people that do this kind of thing are psycho and everything should be done to identify the causes of this, not be so hung up on "where did the gun come from?"