Modern mass killing, usually though not always by shooting, seems to have started (because imitation must start somewhere) with the case of Charles Whitman, who consulted a psychiatrist about his increasing bad temper and desire to shoot people—then killed his mother and father, climbed the tower on the campus of the University of Texas, and shot and killed 16 people at random and injured 44 others. This case has intrigued doctors ever since because at postmortem, Whitman was found to have a brain tumor, which might (or might not) have caused the removal of his normal inhibition against killing. In any case, he had undergone a character change before he killed.
Mass shooters tend to be of above-average intelligence but not high achievers. In a society in which fame, recognition, and success are of capital importance, in which lives of quiet desperation are not à la mode, they often feel deeply wounded, insulted in fact, by the supposed failure of society to take them at their own estimate, to recognize their true worth. They are rarely at ease in their personal relationships, either—but they look outward, not inward, for the source of their failures. The world for them is evil and unjust, and it will get, at their hands, what it deserves. In the process, of course, as another benefit, they will achieve their deserved fame or notoriety. They will be the subject of articles, news items, a Wikipedia entry, perhaps even of books; they will be an object of puzzlement by the learned and the famous. If they go far enough, they might even be mentioned by the president of the United States.