You seem to have a misunderstanding what atheism and humanism are. Atheism is not a religion of its own, it does not have a church or leaders dictating what the moral values of atheists or all society should be. There is really only one thing that atheists have in common - they don't believe in a deity for which there is no evidence. Other than that, they are as varied in their views as the human race itself. Atheists don't set moral values, they expect these to be set by society in a democratic process, and/or to be developed within each human being according to his/her family teachings or personal experiences, guided by self-reflection and input from multiple sources, rather than a few centuries old texts written by unknown authors.

Your statement that "The subsequent conclusion that it does not matter what I do in this life ..." is a completely bogus logical fallacy, strawman, red herring, moving the goalposts . This is your made up interpretation of atheism, it is false and defamatory.

You say "It’s just that there is no pre-packaged moral code that atheists get given when they deconvert. It’s not like a newly reborn Christian who is handed a Decalogue and two thousand years of moral teachings right off the bat. Exactly. We don't want to have our moral values handed to us by ancient priests. Moral values are not zero sum, black or white ideas. They are constantly changing. Moral values of the Bible include support for slavery, torture, misogyny, homophobia, incest, ethnocentrism, war, and murder and fratricide. I don't think atheists have to apologize for rejecting these pre-packaged moral codes.

And your description of humanism is way off base. Humanism does NOT, as you suggest, mean that only humans matter. In fact, humanism works to develop moral and ethical standards that work fully in conjunction with nature. You should not be making judgments about other people when you don't understand what they are about. Before you rant any further about humanism, I suggest you learn something about it. Here is a place for you to start, and after you review, I will expect a retraction of this diatribe:

https://americanhumanist.org/what-is-humanism/definition-of-humanism/