The Oregon shooter targeted religious people, as did the columbine shooters. Their shootings weren't do to religion, it was part of a self serving narrative that cast them as heroes, which can be constructed from any religion or lack thereof.
Timothy McVeigh thought he was at war with the fed, the LaFayette shooter thought he was fighting feminism, the planned parenthood shooter thought he was saving babies, Dylann Roof thought he was saving white women from black men.
It's not about religion, it's about loneliness and narcissism, and it can come from any background.