Must Buddhists be vegetarian ? This is a constant debate among the Buddhists schools. In fact, many modern Buddhists do not know the reason or rational behind this 'rule'. So, what did the Buddha say ?
The Buddha teaches that a person is considered pure & virtuous by his thoughts, speech & actions, and not whether he eats meat or not. A person may be a strict vegetarian (Hitler is a very good example) but still commit heinous crimes. A person may eat meat, but he can still practise the Dhamma diligently.
In the Jivaka Sutta, the Buddha taught that there are three cases in which meat may not be eaten by a monk : Having (1) seen (2) heard, or (3) suspected that the meat has been especially acquired for him by killing an animal (i.e. the animal has been killed specifically for the monk). This rule is called the Rule of Tikotiparisuddha (Pure in Three Ways).
However, the meat of the following ten beings is forbidden to be eaten by the monks : human, elephant, horse, dog, snake, lion, tiger, leopard, bear, and wolf. (Vinaya, Mahavagga, Book 4)
In one of the Sutta, Devadatta, one of the Buddha's disciples, tried to create a schism in the Sangha by suggesting that the Buddha make vegetarianism compulsory, but the Buddha rejected the idea. Why ? Because the Buddha understood that not all landscape can support the growth of vegetables to support the population. When living in places such as mountains and deserts, one has no choice but to eat meat due to the scarcity of vegetables.
According to the Pali Nikaya, the Buddha did not forbid the eating of meat for neither monks nor lay-followers. The Buddha and his Sangha ate meat when they went on alms, and monks are allowed to accept "what has been put in their alms bowl".