personally I do not get the head scratching over this. in retrospect its the perfect storm: a mentally challenged young man, isolated, spending his time immersed in virtual kill reality, hour after hour, day after day, with access to guns...why is everyone asking why? its perfectly obvious why, especially if you throw in the threat of a new life style change that is being talked about as a factor.
A poster gave an excellent FBI link yesterday called "The School Shooter:
A THREAT ASSESSMENT PERSPECTIVE". I read it and it contains multi-tier analyses of assessing threats, and also discusses a 4-prong approach to evaluate how likely a person (generally a student) becomes an actual school shooter.
It addresses the individual's personality, the family, school, and social dynamics. It's a worthy read! Highly recommend

It can help enlighten some of motive behind AL's shooting.
http://www.fbi.gov/stats-services/publications/school-shooter/
from the report:
The student has easy and unmonitored access to movies, television shows, computer games, and Internet sites with themes and images of extreme violence
Signs of serious mental illness and/or substance abuse disorders can significantly elevate the risk for violence and should be evaluated by a mental health professional.
The student is self-centered, lacks insight into others' needs and/or feelings
The student shows an inability to understand the feelings of others, and appears
He avoids high visibility or involvement in school
activities, and other students may consider him a nonentity.
unconcerned about anyone else's feelings
Fascination with Violence-Filled Entertainment
The student demonstrates an unusual fascination with movies, TV shows, computer
games, music videos or printed material that focus intensively on themes of violence, hatred,
control, power, death, and destruction. He may incessantly watch one movie or read and reread
one book with violent content, perhaps involving school violence. Themes of hatred, violence,
weapons, and mass destruction recur in virtually all his activities, hobbies, and pastimes.
The student spends inordinate amounts of time playing video games with violent themes,
and seems more interested in the violent images than in the game itself.
On the Internet, the student regularly searches for web sites involving violence
The family keeps guns or other weapons or explosive materials in the home, accessible to the student.
No Limits or Monitoring of TV and Internet
Parents do not supervise, limit or monitor the student's television watching or his use of
the Internet. The student may have a TV in his own room or is otherwise free without any limits
to spend as much time as he likes watching violent or otherwise inappropriate shows. The student
spends a great deal of time watching television rather than in activities with family or friends.
Similarly, parents do not monitor computer use or Internet access. The student may know
much more about computers than the parents do, and the computer may be considered off limits
to the parents while the student is secretive about his computer use, which may involve violent
games or Internet research on violence, weapons, or other disturbing subjects.
Student's Attachment to School
Student appears to be "detached" from school, including other students,teachers
Unsupervised Computer Access
Access to computers and the Internet is unsupervised and unmonitored
Media, Entertainment, Technology
The student has easy and unmonitored access to movies, television shows, computer games, and Internet sites with themes and images of extreme violence.
Outside Interests
A student's interests outside of school are important to note, as they can mitigate the
school's concern when evaluating a threat or increase the level of concern.
(commentary....if the reports are true, his
only real interest was violent virtual kill video realities)