Somebody ran the manifesto through a plagiarism program and found a guy named Brett Stevens wrote it.
He is a long-term blogger who runs a multi-author blog. He's a critic of the Enlightenment, egalitarianism and multiculturalism. Easy enough to find his blog. To state the obvious: he doesn't advocate for shooting up WalMarts.
Possibly the shooter cut and pasted some of his manifesto from Stephen's site.