Note that a- is a different prefix than anti-. a- means "without", whereas anti- means "the opposite of". So, being a-theistic means that you are "without god(s)". Being anti-theistic would meant that you oppose god(s). An amoral thing is a thing to which morality doesn't apply (for example, weather phenomena are amoral). Anti-moral would mean something that is opposed to morality, which would probably make it immoral (such as a human killing another human that hasn't wronged them in any way).