There are various opinions among the different Buddhist traditions with regards to this question. The Buddha, himself, was not a vegetarian. Traditionally, Theravadin monastics live on alms food; they receive whatever is put into their alms bowl. In other words, they have no control over their diet. However, in the Jivaka Sutta (MN 55), it is mentioned that the Buddha only allowed meat to be taken on the condition that it is pure in three aspects – that the monastic:
- did not see the animal being killed
- did not hear the cry of the animal being killed
- did not suspect that the animal was killed specifically for the monk/nun
Although these conditions technically apply only to monastics, they are often used as a reasonable guide by devout lay people. The Mahayanists relate these three types of “purified meat” (三淨肉) to the nurturing of compassion. If we see the suffering of the dying animal, we should be compassionate towards them and try to relieve their suffering. It is against the principle of compassion if we do not help them, and indeed, even go ahead to consume their meat.
Exceptions have also appeared in Tibetan Buddhism. In Tibet, people traditionally lived as nomads and it was, and probably still is, difficult to grow vegetables in the high altitudes, making it difficult to be vegetarian. Hence, monastics from the Theravadin and Tibetan traditions are generally not vegetarian.
When Buddhism spread to China, the idea of compassion was developed further in the Chinese Mahayanist tradition, and the Bodhisattva vow of not taking meat was strongly emphasised and made compulsory. Chinese Mahayanist monastics are therefore vegetarian, and so are many devotees who have taken the Bodhisattva precepts.
Today, more and more Theravadin monastics are encouraging vegetarianism, and His Holiness the 14th Dalai Lama also encourages Tibetan monastics living outside Tibet, and who have control over their diet, to be vegetarian.
In my opinion, vegetarianism should be encouraged, but not imposed on others.