Take an example: a lost purse full of money is lying on the street and a homeless person is nearby. In the purse you find a business card of a rich businessman, who, in my opinion, would not really miss the lost sum of money What do you do? You can pick it up and go to the police to hand it in. Or you can pick it up and give it to the homeless person. First is dictated by your logic and laws: lost property must be returned to its rightful owner. The later is dictated by your conscience: help the poor man who cannot fend for himself. So what do you do? Be a law-abiding citizen and hand the purse back to its owner? Or giving the money to those who really need it, earning the goodwill of your conscience and the gratitude of the suffering? Well, it isn't easy. Some would do one thing, some other. After all, as Aristotle (384BC - 322BC) said "It is not always the same thing to be a good man and a good citizen". .
People doing different things suggests that people do not interpret ethics and laws in the same way. Or that there is a slightly (or completely) different set of ethics in people's minds. Both things can be explained by differences in upbringing and culture.
So how does one decide whether one's behavior is ethical or not? .
In the case of only one person the answer is easy: that which the person thinks is ethical, is ethical. But what about an event where many different ethical norms are present, such as war? One example can be this: you cannot feed the prisoners of war, so what do you do? You kill them so as to be rid of the "problem"? Or you send them back to the enemy, so that in the future they might kill you? Some culture's ethics would dictate killing as the right way, some would dictate sending back as the only ethical way. However, having ethics at all is better than having none: at least if you have ethics (however different they might be) you have something to base your actions on, and if you have none than you lose a cornerstone of your consciousness, that which defines us as human and that which elevates us to beings above animals.