Animal kingdom can be divided into herbivores, carnivores and omnivores.
What do the philosophers say about the consequences of being herbivores, carnivores(including cannibals) or omnivores ?
How does it affect emotionally and politically? What is the stand of philosophers from ethical point of view?
I'd recommend the works of Peter Singer, who has written extensively on this topic (e.g., Moral Status of Animals).
In general, animals are not held to be moral/ethical agents regardless of what they eat. This leaves us with humans.
For humans, the main debate centers around a a couple key sub-debates:
The rights of animals vs humans, both in terms of survival needs and more aesthetic/quality-of-life issues (e.g., pain, abuse, freedom, exploitation).
The environmental consequences of raising animals for food vs plant sources.
(1) is more primary as it lays the groundwork for where we have the right to put obligations on people's treatment of animals.
For example, in prehistoric times people did not have any concept of macronutrients, nor did they have the luxury of a lot of choice. In these circumstances, it could be argued that their killing of animals for food was necessary for survival; therefore, avoiding killing animals would amount to killing themselves - at best, this is an "even trade", but one could argue that a family of people being saved for one animal is a fair trade.
This is essentially an argument for a moral free-pass for humans due to lack of nutritional knowledge and an inability to act on that knowledge even if they knew it (due to scarcity).
Going to our modern times, these reasons are no longer applicable to a large number of people who live in the developed world. Today, we have a very solid understanding of basic human nutrition and protein needs. In addition, food science has advanced to the point where we can both manufacture complete proteins without animals and know how to combine plant protein sources to create a complete proteins (e.g., grains + legumes). Finally, we also know the relative environmental impacts of raising animals vs plant-based protein sources. Since we no longer have these reasons, it comes down to whether the act of raising animals to be killed for food is inherently bad. Dr. Singer goes into depth on this topic, as does the link above.
The bottom line is that it is hard to argue for eating meat when you have the ability and financial means to avoid it. However, there is huge cultural inertia here and also a lack of overall concern for faceless animals who end up in our supermarkets. This means there isn't a sense of public shame about admitting to eat meat, which seems to be necessary to enact large-scale ethical recaliabration.
Pythagoras, practically the first philosopher (1st to create a school) was kind of vegetarian.
He and his students mainly ate honey, bread and things from nature, avoiding meat. Some say he ate fish rarely. His thoughts regarding what to eat was based in avoiding food that is not "easy" in digestion or has undesirable effects. His main opposition was in eating animals that he considered them like relatives to people, and for this reason he "made" politicians who write the laws to avoid eating animals in the basis that you cannot be unjust to animals if you are "telling" other people how not to be unjust. Although he permitted the non initiated ones to eat animals some times, he did not permit to eat the heart and the brain. These info comes from biographies from 2nd and 3rd cent. AD, since the only original relative phrase from his time is «Μη εσθίειν όσα μη θέμις» which means "do not eat that which is not permitted".
Unfortunately I do not have documentation in English for more info.
