If atheists are right and there is no God, then right and wrong are religious concepts we can dispense with. There is no final judgment, and we are free to act as we like. Stronger people are free to cut in front of any line by virtue of their strength. If someone cries "unfair" then we can say "why is it unfair?" Who decides what is fair? Stronger, bigger animals get first choice of food, is that fair?
If we don't have to worry about God telling us "thou shalt not
" then we can murder, steal, and lie with impunity. No deviant behavior is immoral or wrong, just different. The best you could do is have a majority determine what is approved or unapproved by that group.
Fortunately, God exists, everyone is born with a moral compass, and societies around the world, past and present, value similar virtues. Love, honesty, sacrifice, and courage are traits esteemed by all people. Because God's light is in each of us, Even atheists.