We often ask the wrong or a redundant question which really gets us nowhere in facing our challenges and difficulties.
We see this "chasing of our tail" often in moral quandaries. The temptation is to ask "what is the just thing to do?" as the key to our problems, when in reality we have to give content to what is justice. To ask "what is the just thing to do" is simply to ask "what should I do" in different words. That redundancy gets us nowhere in practical terms.
Christian morality is a morality of self-gift. I like it said better in Spanish: "la teología moral es una teología de entrega." Catholics can find the locus classicus for this idea in the Vatican II document Gaudium et spes, (section) 24, where we are told that the fullness or happiness of human beings lies in the sincere gift of self (look up the original language at the Vatican website).
So "what should I do" or "what is the just thing to do" is now filled with real content: "what does entrega (the sincere gift of self) require?"
Well, entrega means I cannot lie or deceive the other. That takes care of a lot of the commandments, if not all of them. Go through the list and see how many of the Ten Words come down to not deceiving or lying to the other, including the Other who is God. For example, envy is a form of deceiving the other by wishing ill to him. Entrega (I will no longer italicize the Spanish word since I have now explained it) means making oneself a sincere gift to the other in varying degrees. Parents do it all of the time. Parenthood is one major form of entrega, and so Christianity gives us the image of God the Father and also of the ultimate sacrificial entrega of the shepherd for his flock that is pictured by the crucifix.
Etymologically, "entrega" comes from the Latin "integrare" which means, as you can guess, to make whole or complete. Entrega makes us whole and complete, it "integrates" us. Entrega is the core of our human identity. That's the way I read Gaudium et spes, 24. If you want to explain the heart of moral theology, there it is:
"man, who is the only creature on earth which God willed for itself, cannot fully find himself except through a sincere gift of himself."
(Image via Creative Commons License)