Probably this. No atrocities committed by any government were done because the government viewed itself as 'evil' so it must do evil things, the perpetrators always view their crimes as justified for some form of greater good. If one believes a single lie is just a single stepping stone on the path to some grand utopia, or if that lie would prevent some chaos (social disorder, rebellion, etc), some will have no problems telling that lie as many times as they need to.