I think it is partially there own motivation not to believe in evil as a reality, I mean, who -wouldnt- want to live in a world ( wether or not it is imaginary ) without any sort of negative consequence??. . And partially the religious communities moralization of evil, what I mean is compartmentalizing "evil" to a certain set of actions, thoughts, or objects. Instead of communicating WHY certain things CAN be or ARE evil, they simply follow dogmatic advice, Never understanding the nature of evil themselves, then passing it along to others. . . This ( quite understandably ) seems very incredulous to a person who investigates with the question "what is evil" and then they get answers like "sex before marriage is evil" and "becuz God said so" . . I would say the fault for evil being "stealth" falls on both the believers and unbelievers alike.