The question of whether or not vegetarianism is ethical can be approached from so many angles that it seems almost impossible to discuss. 
Many pro-vegetarianism Web sites focus on the conclusion and throw in any argument that will arrive there, including those from ethics, politics, and health. For example, GoVeg.com uses health benefits, animal rights and environmental protection as reasons for following a vegetarian diet. Another pro-vege site (innvista.com/health/nutrition/diet/vresourc.htm) brings economic and human rights issues into the argument by suggesting that meat is too expensive a food source and that large-scale vegetarianism could help prevent world hunger.
In their blind loyalty to the conclusion, they often give each argument equal weight, almost suggesting that red meat’s fattiness is an obvious sign from God of its immorality.
For me, the conclusion is the least important thing. When I hear that someone is vegetarian, I feel that I have found out almost nothing about them. This person could be opposing the sinfulness of killing a sentient being, protesting the modern meat industry or simply trying to lose that extra fifteen pounds. In order to ask the question of whether this person is behaving morally, we must examine the reasoning behind the choice. And in doing so the question itself becomes more intriguing and complex: but complex in an organized, rather than haphazard, way.
Perhaps the most fundamental and most obviously moral question we have to address is that of whether it is right to kill an animal under any circumstance. Most people believe it is wrong to kill a human being, but do not extend the right to life to any other animal. Animals are sentient creatures, however, so they can experience pain. In addition, some are highly intelligent and social creatures that even grieve, in some sense of the word, the deaths of their family members. A contemporary philosopher by the name of Peter Singer argues that many animals are capable of planning and looking forward to the future, which makes untimely death a decided evil for them, just as it is for humans. It is on these grounds that he argues that some intelligent animals deserve a right to life.
But why should a pig have a right to life and not a caterpillar? Both have been living on this earth for millennia. In a sense, doesn’t everything that has made its place here have a right to life? At the same time, however, doesn’t everything have an equal right to kill? This world is based on killing. Everything must kill other beings in order to survive. Every time I kill a spider in my house, I wonder why I should live and not the spider, but I force myself to do it, just to emphasize that the essence of this life is selfishness, is the vivacious affirmation of the self.
The fact that animals do something is not enough to show that it is morally sound, of course. Cats pick up mice and beat them about, torturing them for hours, but I would not therefore condone a man’s pouring salt on a snail and delighting in its suffering. Both the outward action and the inner intention of the subject must enter into the examination. If a man killed and ate a snail in order to feed himself, and if he did so with reverence and gratitude for the life-force he was obtaining, his action would be entirely sound. Humans are the same as other animals in that they need to obtain energy from other life forms, but they are different in that they can empathize with those life forms and can choose to either cause them unnecessary pain or treat them with respect and kindness.
Similar considerations enter into the question of whether it is acceptable to raise animals in industrial-sized feedlots. Does this constitute a reverent use of an energy source or a type of useless torture like pouring salt on a snail? People do not squash cattle into a feedlot and delight in watching them live out their cramped, muddy, stinky lives. In fact, they probably don’t enjoy it much at all.
However, simply not enjoying something while continuing to do it does not make it morally sound. It seems to me that this sort of selfishness goes beyond reverently taking what we need to live vibrantly. It is taking more than we need, or turning our eyes away while a child pours salt on a snail. All of the environmental and economic problems associated with eating meat produced on factory farms really flow from the fact that it is a step beyond beautiful selfishness into the realm of ugly selfishness. The distinction is nebulous, but so are the best distinctions I have come across.
