Below is a post I wrote in response to the following question.
Atheists define good and evil differently, just like theists do. You’re probably familiar with Christians who have different beliefs about what good and evil are than you do. There are Christians who don’t think morality is based on God’s will, and even among Christians who do think morality is based on God’s will there isn’t complete agreement about what God’s will is, specifically. The situation with atheists is a lot like this – lots of different worldviews with no single set of principles between them.
Personally, I think Ayn Rand correctly defined good and evil in her essay “The Objectivist Ethics.” Ayn Rand thought that the purpose of morality was to have the best life for yourself and the people you love. This doesn’t mean exploiting other people, like Nietzsche thought it did, because the only way to become happy is to live a principled, productive life, neither sacrificing yourself to other people nor other people to yourself.
If you’re interested in looking into Ayn Rand’s own summary of her philosophy of Objectivism, you can read it at the link below:
Let me know if you have any questions.