In Japan and other industrialized countries like Germany or UK, just between 50 and 100 people murdered by guns every year, in the States it's close to 20,000 (not sure). This knife story is very sad, very sad, and a big shcok here. But you have everywhere mentally ill peeps, and of course, unlike in the US, there is absolutely no "security" around in Japanese schools. Japan is an extremely peaceful country, with a much, much lower crime rate than the US.
We have to keep the thing in the right perspective: the US is the most brutalized western society, the annual death toll is only comparable with 3rd-world civil-war countries like Liberia.