If there is nothing after this life, then why should one do what is not in one’s own best interest? There is no good or evil because those concepts are just inventions by men used to manipulate the people to act in a certain way.
The worst atheists, Hitler, Stalin, Mao, Castro, Pol Pot, etc., really believed it and thus were not personally constrained by morals and ethics.
There can be no moral absolutes for atheists, no absolute measure of or authority for “good” and “evil”. As you have pointed out morality and ethics are determined by consensus.
Bingo. We have ample historical evidence of what a world without Christian morality looks like. It is deadly and evil.