Karma is not, as the WhyClublet page linked to below suggests, something to do with superstitious reincarnation. Its' simply the effect of a person's approach to their life. Like Aji (see AjiKeshi) it's a self-reinforced process.

For example, let's say that you're shy. Your shyness causes other people to feel uncomfortable around you. Their discomfort makes you uncomfortable ... which reinforces your shyness.

On wiki, BadKarma is most commonly seen in folk who can't tolerate consensual editing. These folk resort to WikiCensorship, which raises peoples' hackles and makes them respond in kind, and so on. Eventually the few folk who have this problem figure out how to deal with it, and become constructive and highly respected WikiZens.

A very few apparently never figure out how to deal. At last count there were 3 of these. I don't need to name them - if you hang out here for a couple of days their identities will be obvious to you.