Collective Karma is a very important theory in Buddhist doctrine. Almost all Karma is accomplished in a collective way, involving more than one person or being. It is almost unlikely that a single person, not dependent on any other, should accomplish a forceful Karma. If you need to accomplish a good Karma, you need to do something beneficial to others. It always depends on the other: if there’s no other, you cannot give, you cannot forgive, you cannot help. It is due to the presence of other sentient beings that you are in a position to do something beneficial or, on the other hand, something harmful. In fact, the accumulation of bad Karma depends on other sentient beings. Whether you are killing or stealing or indulging in sexual misconduct, it always depends on the presence of other sentient beings. If there is no other living being, you can’t kill anyone. When you kill somebody the karmic force is individual as well as collective: individual because the actual act of killing is being done by you alone; in this regard it is an individual Karma. But it is also collective. For example, the person you have killed or harmed might have wronged you in a previous life. Therefore, the act of harming that person is not “free”: the person being harmed has some contribution from a previous lifetime. Or it can be a matter of usefulness: most animals are killed for their meat or their hides, and the perceived necessity for meat and hides are relative to each other and to the killer and to the killed animal, and in this way a collective karmic force is established.

-- 5th Samdhong Rinpoche, Lobsang Tenzin