People don't live or function in a vacuum. They live and behave based on the ideas they conform to.
Don't get me wrong, I agree that the people are responsible for their actions, but we can't separate the people from the ideas. Some people function well with worldviews that can't form a coherent morality system because there is something about those people that makes them predisposed to do good.
Other people, like myself, used the foundational ideas of the atheistic naturalistic worldview to do the bad things that I wanted to and not feel guilty or ashamed about it at all. I could steal, cheat, lie to, and hurt other people to get what I wanted because of "survival of the fittest," it was me or them, or how we said it in the street, "you gotta get it how you live." I didn't rationalize myself into believing that at the time (I was to high to think that deep back then.) It boils down to being a product of a fallen human nature, environment, and ideology/belief system.
I would rather live according to a philosophy that can be the foundation of a coherent morality system. Not one where if the adherents were leaders of the country and they decided that committing atrocities against people was what needed to be done for "the good of the collective/country" then that philosophical system couldn't logically and consistently be used to decry their actions.
That is why I mentioned communist China and Russia, which at the time were avowed atheistic countries. If morality is up to people or the society we can't call what individuals and entire belief systems do "bad" or "immoral."