Most interesting answer to a test question

Students were asked an essay question on public goods and externalities. One student  provided some very notable examples of positive externalities: weapons and illegal drugs.

I sat back, pondering what kind of preferences it would take to imagine that guns and drugs were positive externalities. So far I'm debating if a Libertarian would agree. Most of them deny the existence of most externalities, though they might be sympathetic to the idea otherwise.

A little later, the student gave me a further example: the more crime there is, the more weapons we need, so each additional weapon benefits society. That does sound remarkably like the Libertarian argument for no gun control....

Have I just discovered externalities for Libertarians? Could this be connected to so-called "Libertarian paternalism?"