They believe that if there is no god, then there is no accountability. Since human nature, they insist, is intrinsically corrupt (look at history or current headlines), the tendency will be toward destruction and evil unless there are strict laws and absolute enforcement. We are rambunctious children who need to be broken like wild horses, or reined in and controlled by our wise parents. The fear of punishment and the loss of parental approval provide the necessary moral imperative.
