What is morality/good/evil to an atheist?

[quatona]

What's so wrong with assuming that a common opposition is expressed commonly in the common example of that commonality?

The first problematic aspect: Lack of belief in what you believe in isn´t necessarily opposition.
Second: There is no intelligible reason to assume that the communality goes beyond the part that they have in common.

 

[Gladius]

I want to know what they believe to be the origin of their morality

I assume, perhaps I'm wrong, that most religious people would stem their morality from the various commands of their religion.
I don't need to have a bunch of Christians quoting what Jesus said when I already know.

The way I see it, if morality doesn't stem from God, there are a few ways to answer this.

1) I don't know where morality stems from
2) No such thing as good and evil, only practical outcomes that benefit the species and hence we label them as good; one's that don't, are evil
3) All subjective beliefs that are shared traits within a species that have survived.

I have always thought the ideas or very the concept of good vs evil, stem from religion because I don't think science would ever label anything as good or evil, but rather in their details.

So I'm interested in the connection and the labeling of good and evil by people who don't believe in God.
I can speculate what atheists might believe morality to derive from, or I can ask them, so I choose the latter, but since you asked, I speculated some in this response.

I like your definition #3, with a few minor alterations:

"All subjective beliefs that are agreed upon by a social group".

That's pretty close to where I believe ALL moral beliefs (including religious ones) come from. The history of the founding of Christianity (agreed to by Christians) evidences this. The New Testament is a consensus of human opinion about the Jewish+Christian God.