What role does the human conscience play in developing an ethic?
There are a number of perspective that you can use; as you probably know ethics is a broad topic and there is little agreement at times. In light of this point, let me give you a theological answer on ethics and the important of the human conscience.
First, it is possible that the human conscience is wrong. Hence, it might not be a reliable indicator of what is wrong or right. Just think about mental illnesses. Some people who suffer from metal illnesses commit crimes without a hint of guilt. So, even if the conscience is important it is not the final court of morality.
Second, with that said, the human conscience is also an important indicator of morality. Christian ethics states that there is a sense of right and wrong that people have and to go against this sense is to be morally culpable. Even if a person does not believe this point, there is an insight to be gleaned, which is: to go against your conscience is to transgress an internal filter of what is wrong. This in and of itself is significant.
These two points should get you started in thinking about this rich topic.