AFAIK, in order to believe in karma as such, it's necessary to believe in the Hindu system of reincarnation. In that sense, I don't believe in karma. I do believe that good things tend to happen to "good" people, and bad things tend to happen to "bad" people (which is a more informal definition of "karma"). This is not an entirely unique belief; several religions have some variation of the "Golden Rule". The idea that parts contrary to the health of the whole should be (or will inevitably be) expelled, disabled, or destroyed also exists in various fields at many levels. I don't think of it as any kind of divine justice or other "higher power", but rather as a complex system of cause and effect.