Ethics and law are two very different things. An act can be lawful and yet not good, an act can be illegal and yet good. Rosa Parks broke the law when she sat in her seat. I'm not saying that had she been an atheist she couldn't have acted, merely that you don't obey the law just because it the law.
Plus almost all legal systems have a Judeo-Christian basis thanks to history, colonialism and international trade. Even more than that most are either Roman, English or Napoleonic - so hardly a pluralistic background.
Personally utilitarianism, often held as the secular morality, threatens to go down some really dark paths and has done so. You could easily see how things like eugenics (widely practised in the West until the Nazis put it beyond the pale) became popular, and it was the Catholic church who led the fight against it.
Yes they are very different things, but they don't contradict [1] most of the time. According to legal theory of positivism [2] what is written rule is a set of validated moral laws.
And indeed, as I said my way of thinking gives me huge library that suggest me what to do and what is good and what is bad.
Most importantly I think that human beings ( and animals in that matter ) have really big spectrum of emotions that we carry over thousands of years that are not here today, because we need to fight them.
Fear can save your life sometimes ;
Anger ( adrenalin ) can make you a better fighter in time of trouble ;
Jealousy sometimes give motivation of doing good deeds ;
etc.
If the results of those emotional decisions are not regulated by law and only by moral ( of your cultural ancestors, family, education, etc. ) then I really think it's up to me to decide, otherwise we will all be copies of the moral we inherited and there will be no change at all
The concept of "no good without religion" has a pretty decent intro with both sides at https://en.wikipedia.org/wiki/Secular_morality#Morality_requ...
Personally utilitarianism, often held as the secular morality, threatens to go down some really dark paths and has done so. You could easily see how things like eugenics (widely practised in the West until the Nazis put it beyond the pale) became popular, and it was the Catholic church who led the fight against it.