If atheists reject God, Evil, Good as meaningless and nonexistent, as you have insisted, then the question is likewise meaningless and nonexistent and nothing a theist says will be meaningful to the atheist.
You've walked yourself into a corner.
Let me help you out, atheist Luke Russell, author of
Evil: A Philosophical Investigation summarizes:
When asked to describe wartime atrocities, acts of terrorism, and serial killers, many of us for reach the word “evil”. But what does it mean to say that an action or a person is evil? Some philosophers have claimed that there is no such thing as evil, and that thinking in terms of evil is simplistic and dangerous. In response to this sceptical challenge, this book attempts to show that the concept of evil has a legitimate place within contemporary secular moral thought. It addresses questions concerning the nature of evil action, such as whether evil actions must be incomprehensible, whether evil actions can be banal, and whether there is a psychological hallmark that distinguishes evils from other wrongs. The book also explores issues regarding the nature of evil persons, including whether every evil person is an evildoer, whether every evil person is irredeemable, and whether a person could be evil merely by virtue of having evil feelings. The concept of evil is extreme, and is easily misused. Nonetheless, this book suggests that the concept of evil has an important role to play when it comes to evaluating and explaining the worst kind of wrongdoing.
He summarizes a chapter
1 The Secular Moral Concept of Evil:
This chapter asks whether the concept of evil is exclusively religious or supernaturally loaded, and what it means to say that evil exists. Some atheists refuse to use the word “evil” because of what they see as its religious connotations. Yet many other atheists use “evil” in condemning atrocities such as the Holocaust, and serial killers such as Ted Bundy. The concept of evil, like that of forgiveness, is available for use by theists and atheists alike, in contrast to an exclusively religious concept such as that of sin. Evil exists in the way that courage, malice, or honesty exist; not as a mysterious supernatural force that is capable of taking over a person, but as a character trait and as a moral property of actions. The chapter concludes with an explication of the philosophical method of conceptual analysis, which will be applied to the concept of evil.
I can find 100s if not 1000s of atheists arguing similarly.
But according to you and your definition they are impossible--meaningless and nonexistent.
If the Holocaust was not evil, that was it?