Being in denial and projecting your human flaws on an imagined external evil doesn't actually change anything. The problem with saying that all the world's evil is caused by the Devil is that you are disavowing all personal responsibility and agency in doing something about that evil, and nothing changes. The Devil sure isn't going to. Moreover by refusing to accept your flaws, you refuse to accept those in others. You become judgmental rather than compassionate, rejecting rather than caring. So you end up mired in powerless rage and misanthropy, banging your head against the walls of your tiny universe. Now that's depressing. By accepting that they are flawed, people can strive to overcome those flaws and become better people. And they can accept those flaws in others as they do in themselves, which is the first step towards mutual compassion and cooperation. Basically acceptance of human flaws is the first step to enlightenment as a species.