[edit] And apparently, Hal forgot about me.. heh I think..... Differently
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Martial Arts is not to pick fights. Yes I realize that you don't want it but to defend yourself, and any other martial artist would agree with me on this. Personally, when I first when to a martial arts cool, I felt VERY weird. Just a "How the hell am I going to fit in" kinda feeling. If you can find a school that is affordable (and a good intro deal to test it out), I really would say to try it out. If you get a good school its incredibly fun all the while feeling more confident if something like that should arise.
I don't know you personally so I don't what you're trainng to be. I still think its great stuff to know. I myself am a very passive person. I would do anything I could to not fight, but if push comes to shove.. sometimes you simply cannot avoid having to defend your own. Such as the case you have just been in, sadly enough.
I'm not entirely sure by saying forgoe the karate and continue being who you want to be. I don't think Martial Arts will really change you. If it WERE to change you, it wouldn't be in a negative way. Again when you learn martial arts, the one thing you do know is that you don't use what you know to hurt others. A true martial artist would try to resolve the issue without the violence. This sounds like you if you say you're rather passive at heart.
If you're interested in taking Martial Arts, you should post it somewhere here on TFP and also search it out. There is more than just Karate. I, myself, have trained in Tae Kwon Do. (and a few other things.. my Instructor liked to mix many things into his teachings) There are many different styles and there may be a type that suits you best.