[Frank Ettin]

Any sort of widely available information serves to inform and educate those who might use the knowledge for bad purposes as well as those who will use the knowledge for good purpose. That is the "two-edged-sword" of readily and widely available information.

The major instructors and schools try to vet in some ways those who will have access to the training. As MLeake points out, Massad Ayoob requires certain credentials. Gunsite requires a letter of reference and evidence of a clean background.

By its nature, information that can help you defend yourself necessarily contains information that would help one defeat those defenses. But there's really nothing that can be done about that sort of censorship. And as a policy matter that's a non-starter.

Perhaps what this means is the those interested in protecting themselves need to assume that at least some of those who would do harm have access to the same generally available defensive information, and they will need to find ways to deal with that fact. For example, training beyond what's widely available on YouTube, can be a useful edge.