I'd point out that God's Law is to be followed over man's law, where conflict exists. I'd also point out that everything Hitler did was legal under civil law - so its no secret that man's law can be manipulated so that it ceases to be good and just.
Even for the atheist/agnostic; they should still listen to their conscience over immoral civil laws.
That is a good comeback. If someone does not think that abortion is murder, then I could say that Hitler and his regime did not commit murder because his authorization to kill people was legal under his country's civil law.