I used to teach MMA, and this was always one of the dilemmas I had- was I teaching potentially harmful techniques to unethical individuals?

Sure, if someone illustrated directly that they were a psychopath, I could tell 'em to GTFO- but typically, people put on their best behavior in the gym. There is also the fact that many young people don't yet fully show their later inclinations.

Running a dojo is a business- basically, if someone walks through the door with $ in hand, you don't give them a psychological evaluation before showing them how to choke somebody unconscious or throw an effective strike combo. You assume they are wanting to better themselves and learn slf defense. While I would always throw in ethical aspects to the lessons, especially with kids/young adults, the fact remains... you may be showing a criminal how to more effectively hurt people.