In some states, having proven martial arts ability qualifies your body as a deadly weapon.
If you injure someone, even if they hit you first, you can be charged with “assault with a deadly weapon”. The risk increases if your assailant is part of a certain protected class of “people”.
The rationale is that a professional fighter can subdue someone without injury and any injury caused is unnecessary and on purpose.
Never register your martial arts ability and never publicly boast about your belts, trophies, etc.
All those fell into the lake this summer during the same terrible incident where I lost my guns.
What this guy was doing is a style of kickboxing called Muay Thai that is most definitely the real deal. Loads of conditioning (meaning that not only can he give a strong blow but he can also take it), hitting endlessly until the shins and elbows are calloused and desensitised (meaning he can hit you harder without feeling pain himself), and an aggressive mindset. I have a close friend who also killed a man using his Muay Thai skills when he was robbed at an ATM in Nairobi. He was jumped by two men, and although he had a license for a gun (one needs to have a license to carry in Kenya, which is quite the laborious process) he simply didn't have time to reach for it. With both guys attacking nonstop all he could do is hit back and he fortunately got one guy with a lucky kick (who ran away since a lot of thugs aren't necessarily that brave and expect their victims to play the role) and he chocked the other one (killing him by mistake). He wasn't charges.
You are 100% right on the need to be careful of you know martial arts - especially reality-based martial arts systems. It is all too easy for a jury to say you acted unreasonably, and if it is revealed that you practice Kali or Silat (Filipino and Indonesia arts, respectively, that are the best for using knives), and you used your knife to mess up some guy - you may go to jail. Google Leo Gaje JR to see what Petiki Kirsia Kali teaches for knife attacks - crazy stuff that I can get away with in Africa but will throw you in jail in New York. Or say you know Krav Maga and you kill a pickpocket! Very easy for a jury to say you could have controlled the situation better. The system I do always recommends that I VERY LOUDLY shout that 'I don't want to fight' and 'please don't hurt me' or 'you are scaring me now' so that any witnesses can know whatever happens next is not my fault. It makes no sense to win a fight and then go to jail.
It's a good idea for students and instructors to be somewhat familiar with criminal and tort law that might apply to their use of force, and their teaching.
For instructors, there is a possibility of liability for teaching, or continuing to teach, someone who has demonstrated a propensity for abusing their knowledge. This is not simply theoretical. Martial Arts attracts the best and worst of characters.
Self Defense Law and the Martial Artist by Peter Hobart, Esq.
Learn the Self-Defense Laws Surrounding Deadly Force
Martial Arts And The Law -- Pecoraro's Martial Arts
Martial Arts and The Law
MartialArtCentral.com's Articles
Self Defense Law - Your Right to Self Defense -- alljujitsu.com
The Law of Self-Defense -- by David Dye Sensei
Shuyokan Ryu Martial Arts Center, Costa Mesa, CA