Accidents happen. For example, a simple punch to the chest may be deadly if it strikes over the heart between beats, which may cause the heart to stop. However, this does not in itself make any technique “deadly.” Death or serious injury may result from an activity but that does not mean the activity is inherently deadly. People die while skiing but skiing is not considered a deadly sport.
However, just as firearms are inherently deadly if used incorrectly, some techniques may also be inherently deadly. If you purposefully kick a person on the side of the head with a full-power spinning heel kick, the implication is that you intended to kill or serious injure the person, because this action is inherently deadly. However, fully focused and controlled spinning heel kicks to the head are used every day during sparring sessions with no injuries. What makes a technique deadly is the intention of the technique and the way it is used.
Therefore, any martial art may be ‘deadly” if it is used in a deadly manner. Conversely, any “deadly” techniques may be used a harmless manner. The practitioner of a deadly martial art can easily spar with anyone; the practitioner merely has to use his or her techniques in a safe manner. Any true martial artist has the ability to control his or her actions, such as the amount of force used in a technique, or the area targeted. How else could practitioners of “deadly” martial arts practice their martial arts? If they did not have control of their actions, they would be no one left to train with since they would be killing each other.