I'm working on a story where a girl is concerned for her friend, as the boy shows many self destructive tendencies and she's worried he might be suicidal. One day he invites her to skip school and hang out, so she agrees, hoping to help him with his issues. Next day, while she's at the mall, she learns the school was shot up, and her friend was the one who did it, having invited her out to keep her safe. What I'm trying to work out is, just what are all the common signs that identify both a suicidal person and a mass shooter? And how can I organically convey these signs in such a way that it would be reasonable for the girl to misinterpret her friend's mental state? The most I know is that both tend to display self-destructive behavior, showing an uncaring attitude towards potential harm to themselves, and typically being reserved from others.
I think you'll find that Google is your friend on this issue. I'm sure there's a lot out there pertaining to that issue, given the common nature of the act these days. Try googling 'mass shooter mental state' or something like that, and keep going with what you unearth. If somebody here on the forum has direct experience of this kind of thing, they might be able to point you towards more specific research. But I think you can probably dig up quite a bit of it simply by googling various aspects of the issue and see what comes up. Try to be careful to use only the sources that are based on scientific principles, not the media-friendly hype that can sometimes be around. This is one of those things where you probably don't need opinions; you will need facts and studies. You can go from there with your story.