There can be no justice system without a judge. It's impossible. So either atheists don't want a justice system or they think that the defendant should be able to judge his own guilt.

So do atheists believe that a criminal defendant should be able to judge his own guilt? There's a reason that society doesn't think so even if atheists don't understand why.

Do atheists understand why society doesn't allow each person to make up his own morals? Society does even if atheists don't.

Do atheists understand why lawyers don't advise criminal defendants to be their own lawyers? Society does even if atheists don't.

So there first has to be a judge in order for there to be a justice system. And if there is no judge then there is no justice, period. Therefore since atheists claim that God doesn't exist, then they are claiming there is no justice. So only without God is there no justice because someone has to be able to judge the deeds of humanity in order for there to be justice.

But we do have a judge so there is justice. And who is qualified to judge humanity? Certainly not a human because he is the defendant. So the only justice system has to come from a non-human entity who has shown his power over humanity. Otherwise there is no justice. But I'm sure that those who are judged as guilty don't think that justice exists just like all criminals who don't want to pay for their crimes think.

So since atheists can't admit their own crimes against the ultimate judge who is God, then they are no more in a position to criticize the judge & can no more understand justice than a convicted criminal can and are therefore the last people to criticize the judge, particularly if they claim the judge doesn't exist. But sorry, atheists, even if you argue with the judge, criticize the judge or deny that the judge exists, your punishment will remain even if you're in the dark forever about why.