Department of Psychology
As human beings, we have difficulty accepting random or senseless occurrences. We want to understand why something has happened, and the strength of this desire seems to be proportional to the horror of the event. When a horrible crime occurs, we want to know why. If it was related to drugs or gangs or an armed robbery, I think we find those sufficient reasons. We do not hate the crime less, but at least we think we know why it occurred.
But consider the case of the two boys who walked into Columbine High School in Littleton Colorado and deliberately killed 12 students, a teacher and then themselves; or the case of the 14-year-old Canadian boy who walked into the WR Myers High School in Taber, Alberta killed one and seriously injured another. It is difficult to imagine events more terrible than our young people deliberately killing each other.
What makes it even worse (if that's possible) is that these appear to have been entirely senseless acts. Oh yes, we have heard that they (and others who have committed similar acts) were outsiders, that they had not been accepted, that they were teased and so on. These reports have sometimes turned out to be false. But even if they were true, nothing that was done to these boys or that they experienced even remotely explains their horrendous crimes.
It seems likely that what happened in most of the school killings was that some random combination of events and personalities and opportunities came together to cause the crime. With tens of millions of children in school, it is perhaps not surprising that every once in a while one of them does something terrible. This is not an explanation. You have all probably gone to your doctor...