It is not moral or immoral. Atheism is a rejection of belief in a deity, only. It says nothing at all about the beliefs a person does hold or his/her ethics, morals, or values. If you are a Christian, then you do not believe in Krishna, Allah, Mithras, the Guatemalan Corn god, or Poseidon, the god of the sea. You do not fear these gods because you believe they do not exist. As a Christian, you hold a belief in one God; however, you are an atheist with respect to all other gods. (In the 1st and 2nd centuries, Christians who did not worship the Emperor as God were called atheists by the Romans.) We atheists reject all the gods, just like Christians, but will go one more, rejecting the Christian god as well. That is all atheism is.
I’m stealing this argument.
Only last year I discovered that I had completely lost any connection to the church and organised religion as a whole. I wouldn’t describe myself as an atheist — the term agnostic doesn’t fit either — but this ‘definition’ is something I can stand behind.