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Under an atheistic worldview, you have no objective moral foundation to justify anything as good or bad. For atheism to be consistent, you have to accept that morality isn't objective (therefore, it doesn't exist), so anything you say is a worthless opinion that is a product of a material, chemical, and unguided process. Which is no different at all from the opinions or thoughts of a serial killer or a rapist.
There's a reason why prominent atheists like Sartre, Nietzsche and even Richard Dawkins have admitted that, if atheism is true, there is nothing in the Universe but absolute indifference regarding morality. Then again, most atheists don't like this because it's difficult to think like a psychopath and live like a normal person.