I'm starting to think at least some mass shooters are motivated primarily by the desire to see how it feels to charge up into a crowded public place and stride along, taking out any human being unlucky enough to cross their path. Sure, they will often claim to have deeply-held ideological reasons, but as many have pointed upthread, why shoot up a grocery store full of women and other soft targets? Or a theater or a school, for that matter?
There are far more effective ways to advance one's own beliefs and principles without murdering a bunch of innocent, defenseless people. Ultimately, killers like the Buffalo shooter really don't give a shit who gets hurt.
For whatever reasons, some people are just bloodthirsty motherfuckers. Sadistic, even. But it sounds way less repugnant to say, "I'm doing this to promote a righteous cause!" compared to "I'm a sick fuck who's always wanted to go on a murderous rampage."