A gun is just a tool. By itself it cn't harm anyone. People kill people. Guns are just a tool they use. They could have easily use a car, a knife, a heavy blunt object, kitchen chemicals, and so forth and so on.
The age old NRA mantra. Anyhow, the guns are important because this is an act inspired by media coverage of school shootings.
Not to suggest that guns are more important than the reasons he acted like he did, but again, a bomb would probably not have been the same, bombers are tactical and dispassionate, they plan on not getting caught. I don't recall any instance of a student using bombs against his shool, well aside from the millions of bomb scares conveniently centered around the exam season.