Here is the law of karma is a nutshell: actions that are done with a pure heart inexorably result in the lessening of suffering; actions that are done with an impure heart inexorably result in the deepening of suffering.
Whether we know it or not, when we train in meditation and the Precepts we are expressing faith in the first half of the above description of the law of karma: “actions that are done with a pure heart inexorably result in the lessening of suffering.” When we slip up and allow ourselves to act out of greed, hate and delusion, we create more suffering, and that suffering will eventually lead us back to the realization that only through training in meditation and the Precepts can we lessen the load of suffering. So experience teaches us that we can trust the working of the law of karma in our own lives.
The law of karma applies to everyone, not to just a few scattered individuals. This means that everyone gets the consequences of his or her actions of body, speech and mind. This is a law of the universe and can be trusted completely. If someone is doing something that harms others, it may or may not be possible for other people to intervene and stop the harm being done. But whether or not other people can so intervene, the person who is doing the harm is creating karma that inexorably will deepen his or her suffering, and that suffering will help lead that person—or someone in that person’s future stream of karma—to renounce evil and embrace the Path of the Precepts. It is for this reason that the Buddha predicted eventual Buddhahood even for the most deluded and destructive individuals.
Therefore, experience teaches us that we can trust the working of the law of karma in the lives of others just as we can trust it in our own lives. Understanding this can greatly help us accept our limits and, while being willing to do the possible when it is possible to do some practical thing to prevent others from doing harm, refrain from banging our heads against a wall in a futile effort to do the impossible when it is impossible to prevent harm being done. In this way, we guard ourselves from allowing our own desire to prevent harm from becoming an insistent, obsessive greed that runs away with us, for when this happens we can easily ourselves become the person who is doing harm.
Whatever other people do, we can attend to our own training. When I was a young monk, my master, Rev. Master Jiyu-Kennett, taught us again and again and again: “Just do your own training.” We make our own karma and must carry the consequences ourselves. We can trust this completely.