Adam Lanza had killed his mother before perpetrating the Sandy Hook Elementary School Shooting.There are many reasons for his actions but accordingto his autopsy, the deterioration of his mental health coupled with anorexia and an atypical preoccupation with violence were the primary reasons.
Lanza was suffering from Asperger's syndrome and OCD. He was also a patient of anxiety, hence lonely and isolated. His mother had plans for him to move out of Sandy Hook and buy him an RV. According to the psychiatrist, this must have caused him to go violent and his inability to control his actions added to the deadly presence of a semi-automatic gun caused the entire problem. He targeted Sandy Hook because he was a past student there and hence knew the place. The cause of Lanza committing suicide is still up or debate.