Answer: Killing creates the most negative karma. But if ways of breaking other precepts create equivalent harm, they are the same in negativity. Refraining from killing creates some positive karma but the true opposite of killing is liberating sentient beings, who will otherwise be killed, and from the dining table, by resolving not to eat them and their products. This creates more positive karma.
The most positive karma comes from sharing the Dharma with loving-kindness and compassion to save spiritual lives, as all other physical gifts will expire, including physical lives saved. As the Buddha taught, ‘The gift of the Dharma excels all other gifts.’ This is so as it is only the Dharma leads to Enlightenment, liberation, Nirvana, Buddhahood, True Happiness.
This is why, when liberating lives, chanting of verses on repentance and taking of refuge is also done to connect the sentient beings with the Buddha, Dharma and Sangha. This is done in the hope of restarting or jumpstarting their spiritual lives. In this way, saving both physical and spiritual lives, the most positive karma is definitely created.