The point of this post isn't to convince you to change your diet to one form or another. Instead, the idea is to explore some of the ethical implications of different human dietary choices. It is up to your individual physical needs and spiritual calling to decide which diet is right for you to follow.
Main ethical questions:
Is it alright that animals or plants have to die to sustain and nourish my body?
There is no one answer to this question, but there are many considerations. First, animals and plants will die to sustain something, if not us. We live in a closed system, and we are designed to eat dead things. This means that, whether it's a plant or an animal, we must kill something in order to sustain ourselves. Living on fruit alone is not an option for human beings, and that is pretty much the only food we don't kill when we eat it.
Some people put limits on what they consider it ethical to kill. Pescatarians limit their animal intake to fish and seafood, and are otherwise vegetarian. Of course, we know that vegetarians eat eggs and/or dairy, but not meat and vegans eat no animal products whatsoever. Some more omnivorous people live by the rule that if they have to kill something, it must be eaten, unless it is threatening our lives or property (it's alright to kill mice and termites, for example.) To them, killing is necessary, but should be limited to absolute necessity. Spiritually speaking, there is no one right answer, and each person must come to their own conclusions.
Do I need to consider the fear and other emotions of the things I eat?
Related to the first question for consideration, let's explore the idea of plant and animal emotions. One of the biggest arguments for vegetarian and vegan lifestyles is that animals feel fear when they are taken for slaughter, and a sense of loss when close members of their herd/flock are taken away. Meat eaters shoot back with studies showing plants also experiencing fear and reacting in a way which might be considered emotional. However, since plant emotions haven't been proven definitively or fully explored, let's focus on what we can more easily relate to; the emotions of animals.
In the wild, animals will face death numerous times before they die. They will be chased by lions, cougars, wolves, coyotes, or whatever their natural predators are. They have to struggle for food and water, and often go without. Many starve to death. If they are sick or injured, they must recover on their own or die, often a slow, painful death.
When humans raise animals for meat, they keep other predators away. No denying that humans are a predator, and will eventually eat the animal. The animal is well fed and watered. It only faces the fear of death once, with the exception of a rare wild animal intrusion; even then they have the farmer/rancher to help them. If the animal is sick or injured, it gets veterinary treatment, or is humanely put down when it cannot be saved; rather than a prolonged period of suffering before death.
I am not an animal psychologist, and I have not worked with chickens or milk cows; therefore, I cannot say with any certainty what a chicken or cow goes through in those industries. I've heard from chicken and dairy farmers, and from vegans who oppose those industries, and I just do not have enough facts to comment as to what emotions those animals experience when their eggs are collected, or when calves are taken away so that humans can have the milk. The chicken operations I have witnessed, the chickens didn't seem to care one way or the other about the farmer handling them or collecting their eggs. A vegan might claim it's chicken Stockholm Syndrome, but I'm not an animal psychologist. I do know that modern milk cows often have to be milked because they were bred to produce excess milk.
The topic is food, but I wanted to make a quick note about wool. Wool sheep are well cared for. They are not sheared until the weather is appropriate, because otherwise the rancher would lose all of his/her sheep. They do not mind being sheared, and in fact many sheep rather love it. Like with milk cows and milk, modern wool sheep have been bred to have an excess of wool and must be sheared for their own health. It does not harm the sheep in any way whatsoever.
Do I need to consider the environmental impact of my diet?
This question isn't as easy to answer. The simple answer is, yes. Of course you should consider the environmental impact of your diet. It is common sense, or should be, that we only have one planet, and we must take care of it.
The environmental impacts of different food industries varies greatly based on how the food is raised. Are the plants a monoculture which displace native species? Are the animals free-range, and how much food must be grown separately for them aside from their pastures, and is it grown sustainably? (purely pasture raised animals have no greater impact than the same animals would in the wild.) How many native, especially endangered or threatened, species are killed in the production of my food? The last question is particularly relevant to everyone's diet, as most of us eat some fruits, grains or vegetables.
For some people, the ethical answer is veganism. As noted, vegans eat and use no animal products whatsoever; at least, if they're loyal to their diet. They won't wear fur or leather, and won't eat honey, milk, eggs or any other animal product. They believe no animal should ever die or be "enslaved," (their terminology) for our food needs.
The trouble is, unless one is a gatherer, or knows a farmer who lets the bunnies, mice, and bugs munch away at their crops, selling what little is left; animals are inevitably going to be killed for you to eat. This murder list includes animals, such as birds, which mate for life, and animals with young babies who rely on both parents in order to survive. There is much emotional strife caused in the animal kingdom because of agriculture, especially industrial agriculture. The only ethical answers for vegans are to take an extreme or the middle.
One extreme would be eating only what one gathers from nature. If one is skilled enough to thrive on foraged food, and I'm not sure that's possible most places in the world, then one can mostly, or even completely, avoid killing any animal for their food; either directly or indirectly. Of course, then one has to be sure that one is gathering sustainably, and not robbing the ecosystem of important plants or fungi which are necessary to sustaining something else in that environment. I don't know that this extreme can work for 99% of those who are drawn to the vegan lifestyle.
Another extreme would be to ignore the death and pain caused by slaughter, necessary to agriculture, of mice, birds, insects, and other animals. Of course, it's the fool's path and will eventually catch up with anyone taking it.
The middle path is to accept that everything will die, and that we must eventually kill something to survive. Perhaps the middle vegan path would involve growing more of one's own food and finding more ethical ways to keep pests away; ways available to a small operation, such as good enclosures, which are cost prohibitive to a large operation. It might also involve some careful gathering of food from nature. Eventually, one will have to accept that things will die in order that we have food, and their families will mourn them.
In addition to the ethics of killing animals to protect crops, vegans (and anyone who eats plants at all; all of you) should take into consideration the environmental impact of their food source. Does it use harmful pesticides? Is it trying to work with nature (through symbiotic planting and such) or against nature? How much wild land is displaced? How is wildlife impacted?
In addition to the concerns which a vegan would have, a vegetarian must consider how the animals used for their milk or eggs are raised. Is it raised in a way which is sustainable to the environment? Are they raised in comfort? Are they well cared for, maybe even loved? Is the harvesting, especially for milk, done in a comfortable way to the mammal (cow, goat, sheep, etc.)? Are the animals ever also used for meat, fur, feather, or leather production? The choices of approach are like those which vegans must face; ignore it, go to extremes to avoid it, or take a middle path and accept it to whatever limit one can.
People often erroneously refer to those who also eat meat as, "carnivores." Carnivores eat little to no plants or fungi. Obligate carnivores, such as cats, eat mostly meat and almost no plants. Other carnivores, such as dogs, have a diet which consists of primarily meat, but may also eat a limited number of fruits and veggies. Omnivores, such as humans, require a balance of plant and animal food sources, often leaning more heavily towards plants. Humans cannot survive as pure carnivores, not even to the level of canines.
Of course, an omnivore has all of the ethical considerations of vegetarians and vegans, including environmental impacts of how plants and animals are raised for their food, and must also consider how one's food is slaughtered. Does it suffer before it dies? Is it killed quickly, as to minimize suffering?
Hunting, surprisingly, might be the most ethical means of obtaining meat for an omnivorous diet. Of course, it would mean hunting according to established limits so as not to deplete populations of animals, and only killing what one will eat (including what one feeds to pets and service animals.) Unfortunately, it's not realistic for every omnivore on the planet to become a hunter-gatherer; we just don't have enough wild lands and wildlife left on this planet.
Contrary to the advice for vegans and vegetarians regarding leather, omnivores almost have a responsibility to wear and use leather. Leather is a secondary product from the slaughter of animals for food. By using leather products in place of other fabrics, and it has myriad uses, one reduces one's impact on the environment by greater utilization of a resource; food animals. Of course, my personal opinion is that wearing fur or leather from animals one does not eat remains unethical for everyone.
There are no easy answers. There is no such thing as a human diet which will both meet all of one's needs, and cause zero suffering to the animal kingdom. No one has the moral high ground when it comes to diet, with perhaps the exception of some of the few hunter-gatherer tribes left on the planet; and, not even all of them. The only ethical answer is to minimize one's environmental impact, and to minimize the amount of suffering which must happen for one to eat. You will eventually have to accept the cycle of life and death and your role in it while on this physical plane. However, we can minimize our impact by carefully examining our food sources.
One last note is the human element. No matter what you eat or drink, I think the most ethical responsibility a person has with their diet is to make sure the people who are raising your food (crop or animal) are treated well. Are they paid fairly? Do they use slave labor? Is there a Fair Trade option for this product which will assure that working conditions and pay for the farmers/ranchers is equitable and good? The last question especially applies to coffee and chocolate, as far as food is concerned, but investigating the source of your food will really open your eyes!
Whatever diet you choose, I wish you good nutrition and good health.