You don't know much about libertarians or economics. Of course there is a role to stop those who violate your rights through violence or "bullying"  and certainly slavery. It is laughable that you would suggest a libertarian society would slide toward slavery. As far as making bad actors, good actors Adam Smith answered that question ages ago. What is the common good? How do you measure it, empirically quantify it, or do you simply mean someone's opinion of the common good.