It is interesting that you have brought this up, as it has been the source of some contemplation for me of late. Basically, I have begun studying the Buddhist/Yogic doctrines, including the Laws of Karma. It seems quite clear in these teachings that Karma is produced as a result of actions of body, speech and mind. The laws state that good things can only come from a good source, and bad things only from bad. Wishing harm upon someone is not as bad as killing them, but it still has negative karmic consequences.
During the course of these studies, then, I have begun to question the methods and activities of martial training. Even in the most honorable of bases (self defense, self development, etc.), we walk a thin line with respect to karmic consequences. In my study of CMA (Mantis Kung Fu), we learn that forms and techniques should be practiced with intent in order to get the most benefit. That intent is usually one of harm to another. A form practiced without appropriate intent is considered empty and lacking. In practice (chi sau, sparring), there are invariably bloodied noses, fattened lips, etc. Once again, bringing harm to others with intent (even if it is softened by the lack of anger or aggression, which (unfortunately) is not always the case).
The question then becomes, if you believe in the causality of karma, what does one introduce into their life by the pratice of most martial arts? Is there a method of practice which will still gain the positive results of martial arts without incurring the negative karmic consequences.
For myself, I have not determined what the answer may be. Until I have, I have actually chosen to stop training actively in kung fu, as I have found my forms empty and without intent in my current state of mind. I have also taken up the practice of Aikido to explore the concepts presented within that art, although I am not yet convinced that the end result is much different. At least in Aikido, all practice is done in a cooperative manner without the trappings of competition or conflict (having predefined uke and nage for each interaction). Of course, that is one of the common criticisms of the art as well. It seems a high goal to seek the skills which would bring about conflict resolution in a "harmonious" manner, even when things have escalated to a physical level.