He swings at your head, but you evade the blow and apply a wrist lock. While you try to disarm him, he resists and breaks his own wrist. You would stand a much better chance defending your actions in court because he threatened you with a deadly weapon. Finally when you tried to disarm him, he resisted, and his own force caused his wrist to snap. All fighting disciplines include techniques that can be deadly.
Most instructors reserve such techniques for their advanced students.
Unfortunately, some know little about the law and fail to explain what deadly force is or that it should be used only as a last resort. Remember that most martial arts techniques were devised long ago when mortal combat was a way of life. We no longer live in such times. We have legal and moral responsiblities to refrain from unjustly using our skills. If you ever find yourself in a situation where you must use force to defend yourself, it is important to distinguish between self-defense and aggravated assault.