We have a good idea what motivates them. Here's some quotes from professionals:
From Katherine Newman, a sociologist at Johns Hopkins University: "In general their social experience is not one of easy incorporation, ...Rather than wanting to be alone, many mass shooters have a history of struggling to connect. They experience rejection by their peers or they draw back from potential friendships, assuming they'll be rejected if they try. They believe they're perceived as insigificant."
And from Dr James Knoll, forensic psychiatrist with expertise in mass murders:
"The mass murderer is an injustice collector who spends a great deal of time feeling resentful about real or imagined rejections and ruminating on past humiliations. He has a paranoid worldview with chronic feelings of social persecution, envy, and grudge-holding. He is tormented by beliefs that privileged others are enjoying life’s all-you-can-eat buffet, while he must peer through the window, an outside loner always looking in.
"Aggrieved and entitled, he longs for power and revenge to obliterate what he cannot have. Since satisfaction is unobtainable lawfully and realistically, the mass murderer is reduced to violent fantasy and pseudo-power. He creates and enacts an odious screenplay of grandiose and public retribution. Like the child who upends the checkerboard when he does not like the way the game is going, he seeks to destroy others for apparent failures to recognize and meet his needs. Fury, deep despair, and callous selfishness eventually crystalize into fantasies of violent revenge on a scale that will draw attention. The mass murderer typically expects to die and frequently does in what amounts to a mass homicide-personal suicide. He may kill himself or script matters so that he will be killed by the police."