To what extent do you feel that human beings need rules in order to be moral?

This question is closely related to the age-old “nature v. nurture” debate. Do people behave the way they do because of rules imposed on them by society, or are they obeying innate (from the inside) laws instilled in them from some other source? If there is a god who instills a sense of morality into our beings, then we are not wholly dependent on rules.
Since your question asks for an opinion, I'll tell you what I think, and why.
I believe that our moral sense comes from a combination of society's rules and something that is imprinted in our minds or hearts by God. If we did not have an innate sense of right and wrong, we would live in a society in which life was much more of a survival-of-the-fittest affair. Granted, the fittest certainly do fair better than the weakest members of society, but for the most part everyone is cared for to some degree. Social programs that feed and house the poor may be controversial, but they do point to our desire to at least do something for them.
Imagine a world in which no one had a pre-programmed sense of right and wrong. Why would anyone do anything for anybody else unless it was in their own best interest? Instead, we live in a world where we feel compelled to make the rules that protect the rights of others, even sometimes at our own expense.
We also live in a world in which nearly every parent willingly sacrifices their own gain for their children's benefit, without hope of a material reward. People may scoff at that and say, sure, that's because we instinctually love our offspring. But why do we possess that instinct? It must have been put there for a reason.
Likewise, why do we have a conscience? We know when we have done something morally wrong. Our conscience won't let loose of us—even if we try to deny it ourselves.
But there is another side to this issue. As a teacher I see all kinds of kids every year. A few come from unfortunate home situations in which they don't learn as much about the rules of moral behavior as others. Sometimes these kids seem more likely to violate moral standards than others. This suggests that it is still important to create and use rules to build on the morality naturally occurring within us.
Finally, I don't think we could even create rules if we didn't have a sense of morality to start with. Why would we want to? Why would we care? But it's also true that if we suddenly lifted all of our laws and rules, we would undoubtedly have many immoral things happen—like the looting we see when a government falls apart.
So the argument goes around and around, and that's probably because, to finally answer your question directly, we need rules to behave morally, but we also need a sense of morality with which to craft our rules.