Shootings at Universities...

Recently there has been shootings at a couple of Universities which ended up with victims dead. I just have that question, of how can a person do that action of killing people, and for what exact reasons. Based on interviews from news, Illness has to be a big part in why people had chooses those paths. In Washington at Foss High school a student was shoot several times by a non-student who was thought to be incorporated with a gang, but later on was said to be suffering from a mental illness. The student from Virginia tech who had conducted the Virginia Tech Massacre was as well said that he had an anxiety disorder since middle school which could of been the trigger to his actions on April 16, 2007. The shootings at schools are becoming more common. So I want to end with a question: "Why would anybody think of such a thing?"