Population pressure and the stress of modern life may cause an increase in violent tendencies.
The urban environment is the incubator for all sorts of undesirable behaviors. However much this atrocity disgusts us, he may actually consider himself a hero. This is common among those who are referred to by the popular slang, going postal.
In his tortured mind, he may feel he was battling against impossible odds. It is not unusual for some individuals to believe that the entire fate of the world rests in their palm. In the end, our subject displays all the classic symptoms of a paranoid delusional.
We may never know exactly what set him off but, rest assured, we will have plenty of time to study him.