Is something is right or wrong independently of whether anybody believes it to be so? For example, is killing innocent people for fun objectively wrong, or is that just a subjective opinion? Was the holocaust wrong? Would it be wrong even if the Nazees had won and successfully brain-washed everyone that it was alright? Or are these merely social conventions, and perhaps society in hundred years could be in favor of murdering infants, and nobody in today's society could claim it is "evil", because there's no objective morality? Or if there is, whence comes the objective morality, if not from God?
If you don't believe in objective morality, then morality is just a social custom – or what is socially acceptable at the time. Hence it is not morally "wrong" to kill babies, it’s just socially frowned upon. How then can you condemn anyone for deeds like murder or rape if it's nothing more than your own personal opinion, a mere social custom, you personally subscribe to?
That becomes a non-question if you believe in God and that he decides what is moral and what is not, so it's a question for people who have no faith.