Criminal laws prohibit (or require) an assortment of activities. If caught disobeying the criminal law, the lawbreaker is punished by the government. Criminal laws are usually written by legislatures and contain two types of crimes: 1) crimes that harm victims (such as murder, assault and theft); and 2) crimes that harm the person or property of no one, except perhaps the criminals themselves.
The latter type outlaws conduct such as drunkenness, gambling, prostitution, drug use, or anything that requires a license when such license has not been granted. Ultimately, a crime is whatever the government says it is.
A recent and popular trend is to make it a crime for anyone to fail to inform the government if they believe someone else is breaking the law. This trend, now firmly established regarding a few especially despicable crimes such as child sexual abuse, may spread to cover other crimes—and eventually all crimes.
In a nation where informing for the government is mandatory, when you “see something,” you “say something” . . . or else. Such a society would see the full blossoming of the police state and, like the 20th-century residents of fascist and communist states, a healthy fear of one’s neighbors, friends and even family could become a valuable survival skill.
In a libertarian society, the maximum role of the state would be to protect life, liberty and property. The use of defensive force is always legitimate, but the non-aggression principle limits the criminal law to things like assault, murder, theft, fraud, trespass and the like; that is, crimes against persons or their property. As Saints Thomas and Augustine taught, [here, here, and here] the law should not criminalize moral vices or other disapproved conduct, where such conduct does not directly harm others.
A libertarian criminal code would be compact and intuitive. Its function is to ensure peace and tranquility, not to make virtuous people. Instead of needing a lawyer to explain it, children would understand it by the time they were two years old.
[excerpted from Chapter 8 of Free is Beautiful: Why Catholics should be libertarian]