A good book that addressing the source of morality - The Righteous Mind. The idea is that human morality has an evolutionary basis, but those instincts are often applied now in ways that are not logical or reasonable. Also that there are, I think, 6 basic tenets of morality. One of those is “care for others” (empathy) and liberals tend to value that above all others, while conservatives tend to more equally weight all of the different types of moral values, including those rooted in loyalty and “yuck.”

some of us are more ethical than others, and my observation tells me that theists find more ways to justify bad behavior than atheists.

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Agreed. My experience with theists is that they cannot comprehend that someone can have a moral compass without believing in God.

I agree, we are a product of millions of years of evolutionary programming - I believe man is basically good but he needed an "evil" or "mean" side when he had to kill or do something unpleasant - but as men we had to do those things - to protect our families, or ourselves.

No god necessary. Social animals (not just humans) often (not always) exhibit behavior that embodies morality.