I currently practice karate. No, you needn't worry. I'm only a white belt, the lowest rank in karate, so I'm not dangerous.
After becoming involved with our club I was struck by the incongruity between the image I had of traditional "martial arts" with what I was experiencing in practice. I expected that the martial arts was a peaceful pursuit. The reason to study martial arts, such as karate, is to learn how to not fight, to learn to have peace, etc. Yet in our karate club, like other karate or martial arts schools, we learn how to hurt people. We learn techniques to use that are oftentimes meant to hurt, possibly used to kill, and we learn places on our opponents' bodies to use these techniques. We spar with each other, after which I often have bruises on my shins. (Nothing worse than that, thank goodness. My club is made of great people who care for each other and try to prevent any serious injuries.)
I don't fault karate for this incongruity. It's the nature of self defense, just as it is the nature of our communal defense (e.g. military), that you learn to hurt the other person. With most styles of self defense, anyway. As long as defending one's self is a part of achieving and maintaining peace training for and practicing conflict will be a part of it.
I find that because of my karate training I think about getting in fights more now. I think about what I carry on my person that I can use as a weapon if I find myself in a fight situation. Where should I hit, how should I hit, what's the best way to "end the fight"? These are all good questions when one is studying how to defend one's self. At the same time I think there are drawbacks to having these thoughts. I feel more aggressive (though I haven't gotten and don't plan on getting into any fights). I start seeing people as opponents, even though it is just in a "sparring" way rather than a "fight in a dark alley" way. These are not peaceful thoughts.
The way I see it, there are trade-offs in the study of karate and other martial arts.
Not all martial arts have the same approach toward self defense. Aikido seems to more closely match my previous expectations of martial arts with respect to learning how to not fight and encouraging peace with others. I understand there are different schools of thought within aikido, but in general the goal is to (quoting Wikipedia) "defend themselves while also protecting their attacker from injury". The founder, Morihei Ueshiba, seems to have greatly cared for the people in our world and desired peace among us.
I recently read a blog post that pointed to a story told by Terry Dobson, a well-known American aikido practitioner who was able to study under Morihei Ueshiba. His story is profound, and tells of what I consider to be a truly Christ-like art.
THE TRAIN CLANKED and rattled through the suburbs of Tokyo on a drowsy spring afternoon. Our car was comparatively empty - a few housewives with their kids in tow, some old folks going shopping. I gazed absently at the drab houses and dusty hedgerows.
At one station the doors opened, and suddenly the afternoon quiet was shattered by a man bellowing violent, incomprehensible curses. The man staggered into our car. He wore laborer’s clothing, and he was big, drunk, and dirty. Screaming, he swung at a woman holding a baby. The blow sent her spinning into the laps of an elderly couple. It was a miracle that she was unharmed.
Terrified, the couple jumped up and scrambled toward the other end of the car. The laborer aimed a kick at the retreating back of the old woman but missed as she scuttled to safety. This so enraged the drunk that he grabbed the metal pole in the center of the car and tried to wrench it out of its stanchion. I could see that one of his hands was cut and bleeding. The train lurched ahead, the passengers frozen with fear. I stood up.
The Rest of the Story