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.