An fear of weapons is a symptom of a more complex condition.
As much as Psychiatrists want to pound away at the father of Psychiatry, he had tremendous insight. We live in a society where people are encouraged to remain in an immature nymph stage of development and never really achieve independant adulthood without somehow being firmly attached to the governments umbilical cord.
When you have sexually immature adults you have problems and symptoms such as a fear of weapons.
Also mental illness may only be part of the equation. Religion whether we like to discuss it or not may have played a part. It is known that Adam had a website devoted to devil worship.
Near the bottom of the article is where they bring their argument around to Adam Lanza and what some of his acquaintances knew about his online activities.