This is a dishonest argument. The argument provided is correlation does not equate causation. Just because there have been atrocities committed by atheists, does not mean atheism is inherently immoral. The same is true for Christianity. You can judge Christianity by a Christian if that Christian is a model of the religion – which I guarantee they’re not.
Take terrorists in the Middle East, are they a good example of Islam? A billion Muslims say no. However, if you choose to judge Islam based on these few individuals, you get a distorted picture and false narrative.
Further, atheists derive a moral framework under the belief there is no God. Christians derive a moral framework from the belief there is a God. These both have consequences – and can both be judged.