In response to undead's comment:
Those diseases don't really have much of a correlation with mass shootings, especially ADD.
Aspergers/Autism can result in singular episodes of maladjusted response to a situation at the time and place of the situation, but not a calculated plan to go kill a lot of people somewhere else.
ADD? For one thing, I have serious doubts about whether most people diagnosed with it even have a disease, rather, I think it's more the intersection of less discipline and the fact that humans didn't evolve to sit infront of computers and chalkboards all day.
The sort of mental illness that can result in mass shootings needs to be one capable of creating a delusion (as opposed to a disorganized schizophrenic break from reality): A false fixed belief system.
Something that allows the actor to act in a way that would be rational if, in fact, his beliefs were true (ie, that the theatre was full of invading aliens that needed to be stopped and no one else could do it).