One of the problems with religions as belief systems is that they explicitly or implicitly support the false and pernicious notion that the content of a person’s beliefs strongly determine their moral character and behavior. The once-common expression, “I am a god fearing (wo)man,” is meant to convey “I believe in the doctrines of Christianity, therefore you should trust me as a good and moral person”. This is nonsense so obvious that most theists today place little emphasis on it, or express it in watered-down, No-True-Scotsman, or metaphysically obtuse forms (Satanic corruption, for example).
But they still tend to believe it. So do atheists, about ideologies they do have. This comes out implicitly in statements and jokes of the form: “Of course person X who did bad thing Y is a bad person, after all they are part of not-my-group group”. The difference is that nontheists have one less major hook for this tribalistic, harmful type of thinking to catch on, and that the notion is not usually a tenet of secular ideologies; For many religions, it is hard-coded.
As an atheist, I don’t have to think atheism makes anyone a good person— it doesn’t. That’s why I don’t need to get defensive when an atheist does something terrible. Morality, justice, and the determinates of a less violent and more stable society (or individual) are entirely different subjects from atheism. True, these things interact, but there’s probably almost zero predictive power of religious orientation alone versus something like violent behavior once you control for all relevant variables (SES, political orientation, education, etc..,).
What about the moralistic -isms like humanism and feminism?
Do these make people more moral or more ethical actors by way of accepting their tenets? Probably not directly. We know this because there are plenty of examples of fervent promoters of these -isms who have acted awfully. So many that it strains belief that there is any strong effect distinct from the opposite causal direction (good people gravitating toward the moralistic ism). I do believe these tend to make society better and rightly claim responsibility for social progresses and I want you to notice why that’s probably true: they are often corrosives against tribalistic us-them thinking that undergirds prejudices.
Secularism bestows the opportunity (not guarantee) for the same benefit. Atheists, you do not need to think non-belief, even in bad ideas, assures a person is good or is anything like yourself. That is antiquated nonsense. You also need not be defensive when others suffer under the delusion that it does. You do yourself no favors by rushing to defend “atheism” by condemning murderers, because it means you tacitly accept the false, pernicious premise. It is as if you are saying, “No, person saying atheism/ists are bad, atheism makes people good because look at us being good actors!” No. Stop. Instead, calmly call the person on their mistake: please provide evidence that fervent atheistic belief is correlated with violence or hate. There is no such evidence because it is not true. Further, you’d be well advised to avoid making the same mistake and looking like a fool or a hypocrite by saying that a major religious orientation has the same effect.