Liberating living beings can also prevent some evil karma. Take the example of freeing one fish. First, if the fish vendor sells the fish to a restaurant, he will have committed karma of killing that fish. By buying the fish from the vendor, we stop that from happening. Second, if we do not buy the fish, the cook at the restaurant will kill it. We prevent the cook from committing that karma with our purchase of the fish. Third, the customers eating the fish are also guilty of killing. By buying the fish, we prevent karma of killing for the third time. As the fisherman would not know at the time of catching the fish if it was to be liberated or killed, his evil karma, if any, may not be prevented by our purchase. But the other three can all be avoided. When liberating beings, to make every endeavor not to hurt them as well as the feelings of other people is in fact benefiting sentient beings already. This manner of liberating living beings constitutes the perfection of discipline.

~Depicted from THE RIGHT VIEW - Liberating Living Beings