Karma is essentially the worst sort of belief, in that superficially it seems to reward good behavior and punish bad behavior. This appeals to a lot of people because of its clear "carrot/stick" mentality and overall simplicity. This is the "Just-World" hypothesis.
In reality, this kind of thinking means that if you believe in Karma, you can ignore less-fortunates because clearly they screwed up their lives on their own. Conversely, you can get away with being a jerk to people, because they probably deserved it anyway. It is a self-reinforcing bias of the worst sort.