What this post is: an elaboration of two of those Buddha quotes:
Monks, these four types of karma have been directly realized, verified, & made known by me. Which four? There is karma that is dark with dark result. There is karma that is bright with bright result. There is karma that is dark & bright with dark & bright result. There is karma that is neither dark nor bright with neither dark nor bright result, leading to the ending of karma.
Anguttara Nikaya,AN 4.235, PTS: A ii 235,Ariyamagga Sutta: The Noble Path,translated from the Pali by Thanissaro Bhikkhu
This quote shows two things: first ‘bad karma’ isn’t such a strange concept to original Buddhism as many modern Western Buddhists claim.
Second, it’s about the effect of actions (and thoughts and words). Those who’ve thought about karma a bit, often come up with this question: how about actions that have a bad motivation, but a good effect. Say a celebrity sponsors some good cause, mainly to beef up their own reputation… The effect is still mainly good, one would say: that cause gets both money and attention. And that attention is bound to bring in more money. So where’s the negative effect? Well, I think the negative effect is in the cynicism people feel towards this sort of thing. We become good cause jaded.
So this is an example of ‘There is karma that is dark & bright with dark & bright result.’ This quote plainly says that there IS no such thing as karma that is good, but doesn’t have a good effect. Cause and effect are, in Buddhism, clearly linked. And in most cases of course the cause is mixed. Both motive and method of action is usually not perfect, so the effect is likely to be mixed as well.
Another Buddha quote with the same message:
So, Ananda, there is karma that is incapable (of good result) and appears incapable (of good result); there is karma that is incapable (of good result) and appears capable (of good result); there is karma that is capable (of good result) and appears capable (of good result); there is karma that is capable (of good result) and appears incapable (of good result).
Majjhima Nikaya 136: The Great Exposition of Kamma (Mahakammavibhanga Sutta)