When you try to codify ethics, you end up getting the opposite of what you intended because then "might makes right". If a law does not protect you from theft or violence, then you are screwed. But if we are to teach people about having ethical standards such as "it is wrong to steal or initiate violence", then with time, people will stop. It won't guarantee that people will have ethics, but we always need to go in that direction. So isn't it better to live our lives from ethics instead of laws?
Also, as for your idea that if people taught that violence and theft are wrong, then after a while they'll stop: Again, the Bible tells us of a time when people did not stop. They knew it was wrong to behave as they were, for even as some people stood up to them or tried to talk them out of their behaviors, they preferred to continue to sin. This example is that of the cities Sodom and Gomorrah.
Ethics usually vary from culture to culture. You could even have different ethics than that of your next door neighbor. So the problem with living just by ethics comes into play, for if authorities were called, then whose ethics would they go by to decide on who's in the wrong? The person who called, the person doing the action, or the authorities'?
In the Bible, we have a good example of God's Law (and ethics) versus the people's ethics. The Israelites had grown up in Egypt and taken on their ethics as their own. But God's ethics are different, and so His Law, presented by Moses, set the standard and rules to live by. This Law also had the purpose of training them up into what it meant to be God's people, as well as to prepare them for the coming Messiah. And the problems they got into with God were always due to their own interpretations and decisions of ethics, thus causing rebellion against God's Law and ethics.
The book of Judges actually talks of a particular time when people did not live by the Law of God, but by their own ethics, or the ethics of the land. It even says in most chapters that there was no king, and so everybody lived as they saw fit. But even though their culture formed their ethics, they weren't the ethics of God, the creator of everything that exists, and they went against His Law. So even though they saw it as right, it was counted as sin.
I do agree with you though on the fact that sometimes laws are created from ethics, often from the experience of needed change. For example, cars driving too fast around schools, a kid gets hit, somebody realizes they needed to make a law saying that cars have to drive real slow around school zones. But in terms of God's law / ethics, He's already thought them all out...no need for change (especially since they've been fulfilled in Jesus Christ).