A fascinating thought.
Why is it that man has such a strong sense of right and wrong? To the point that even when someone breaks their promise we get upset. But if we break our promises we always need to find an excuse as to why we broke our promise.
Is there a standard outside of human opinion that all humans have a similar opinion on?
I'll let Lewis illustrate on a couple of points:
The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other. But the standard that measures two things is something different from either. You are, in fact, comparing them both with some Real Morality, admitting that there is such a thing as a real Right, independent of what people think, and that some people's ideas get nearer to that real Right than others. Or put it this way. If your moral ideas can be truer, and those of the Nazis less true, there must be something-some Real Morality--for them to be true about. The reason why your idea of New York can be truer or less true than mine is that New York is a real place, existing quite apart from what either of us thinks. If when each of us said 'New York' each means merely 'The town I am imagining in my own head,' how could one of us have truer ideas than the other? There would be no question of truth or falsehood at all. In the same way, if the Rule of Decent Behaviour meant simply 'whatever each nation happens to approve,' there would be no sense in saying that any one nation had ever been more correct in its approval than any other; no sense in saying that the world could ever grow morally better or morally worse. - Mere Christianity
Or did morality (if you believe in evolution) come about through evolutionary developments?
(I hope you don't mind me quoting Lewis all the time, I am fascinated by so many interesting insights of his) :)
Note: C.S. Lewis believed in evolution.
Supposing you hear a cry for help from a man in danger. You will probably feel two desires--one a desire to give help (due to your herd instinct), the other a desire to keep out of danger (due to the instinct for self-preservation). But you will find inside you, in addition to these two impulses, a third thing which tells you that you ought to follow the impulse to help, and suppress the impulse to run away. Now this thing that judges between two instincts, that decides which should be encouraged, cannot itself be either of them. You might as well say that the sheet of music which tells you, at a given moment, to play one note on the piano and not another, is itself one of the notes on the keyboard. The Moral Law tells us the tune we have to play: our instincts are merely the keys.
Another way of seeing that the Moral Law is not simply one of our instincts is this. If' two instincts are in conflict, and there is nothing in a creature's mind except those two instincts, obviously the stronger of' the two must win. But at those moments when we are most conscious of the Moral Law, it usually seems to be telling us to side with the weaker of the two impulses. You probably want to be safe much more than you want to help the man who is drowning: but the Moral Law tells you to help him all the same. And surely it often tells us to try to make the right impulse stronger than it naturally is? I mean, we often feel it our duty to stimulate the herd instinct, by waking up our imaginations and arousing our pity and so on, so as to get up enough steam for doing the right thing. But clearly we are not acting from instinct when we set about making an instinct stronger than it is. The thing that says to you, 'Your herd instinct is asleep. Wake it up,' cannot itself be the herd instinct. The thing that tells you which note on the piano needs to be played louder cannot itself be that note. - Mere Christianity
If there is no right and wrong ultimately then how can there be right and wrong? If it is the choice of humanity then it comes down to opinion. It would be merely my opinion to say that Hitler was "wrong".
R.C. Sproul Jr. stated that if there is no centre mark to aim for then how can you tell how far off the mark you are?
I remember a story he once said about when he was lecturing english classes. He stood up against the idea of relativism and stated that there was a real wrong and a real right.
A person in charge of him suggested that his career would not go far if he continued to talk like this. R.C. replied asking "Are you trying to say that it is "wrong" to say that there is a right and wrong?"
The person asked him to leave...
Without God there can be no right and wrong except for opinion. The person that will win their version of right and wrong will be the person with the biggest stick.