Another mass gun-killing in Illinois

I was startled to learn of another university killing, and that at Northern Illinois University, where my mother went. What is it with these killings of the young by the young, Columbine, Virginia Tech, NIU? To my knowledge, they've all been in mid-sized towns (Columbine, CO, twenty-five thousand; Blacksburg, VA, approximately forty thousand; DeKalb, IL, forty-two thousand), places that seemed, like my own hometown actually, busy and thriving but relatively safe. That is, these aren't urban centers, and the victims are generally white. I'm unsure what to think.