Good and evil, absolute right and wrong do not exist in a universe of blind physical forces. Shit just happens, and humans have to create their own value system, assuming they even care to.
Murder to Stalin and Mao was a very good and advantageous way to live a flourishing life. They and their inner circle did quite well with lives based on murder.
Murder to Reverend Martin Luther King was objectively evil.
The 20th century's most famous atheist Albert Camus stated that atheists ultimately realize the universe has no inherent meaning, and they have to turn around and borrow religious values, just stripping them of their religious context.
Camus and the other existential atheists thought that was inauthentic, and they insisted that people had to recognize there was no objective moral truth and that individuals have to create their own value system.