I know that people tend to wax philosophical over morality and ethics but I think, under normal circumstances, the Golden Rule is all one needs to be moral: "Do unto others as you would have them do unto you." As we mature, human experience and empathy naturally leads to an understanding of morality -- we know what hurts us, so we know what hurts others. It's really very simple: don't cause unnecessary harm (including to oneself).
Unless you're autistic or raised by jungle apes, you know right from wrong. Any religious claim of moral authority, or scriptural claim to bequeath morality, is bogus.
Although most of our choices are pretty straight forward, there are also many difficult moral choice to be made. Abortion, social policies, capital punishment, euthanasia, declarations and prosecutions of war . . . there are many moral conundrums in life.
It seems to me that many moral conundrums involve the value of human life. But what IS the value of human life? Humans seem rather cavalier about it. We kill each other in violent crimes, wars, capital punishment, suicides and vehicular collisions. Sometimes killing is justified. War can be necessary. Euthanasia can be humane and merciful. Even capital punishment is popularly supported.
So when and how do we uphold the value of human life? I think the answer must be, "When we can, as best we can." In war, this means taking all reasonable efforts to prevent civilian casualties. In abortion, crime and other social policies, this means education that emphasizes the value of human life so that people will think twice about taking or diminishing the life of another.
If the value of human life is relative, then we need to make people mindful of their own value in a relative system. Then the Golden Rule will enjoy a finer focus when choices aren't so simple.