Pirkei DeRabbi Eliezer 20
R. Eliezer ben Yaakov said: From the skin of the snake that the snake shed God made for them garments.
Literal Blood Thirst
You are permitted to make use of the living creatures and their service, you are allowed to exercise power over them so that they may promote your subsistence; but you may not treat the life force within them contemptuously and slay them in order to eat their flesh; your proper diet shall be vegetable food. It is true that the eating of flesh is not specifically forbidden here, but the prohibition is clearly to be inferred. ...Apparently the Torah seeks to convey that in principle man should refrain from eating meat, and that when Noah and his sons were granted permission to eat flesh, this was only a concession subject to the condition that the blood was not to be consumed. This prohibition implies respect for the principle of life (‘for the blood is the life’), and it serves also... to remind us that rightly all parts of the flesh should have been forbidden. It behooves us, therefore, to eschew eating at least one element thereof in order to remember the earlier prohibition.
Umberto Moshe David Cassuto 1883-1951, Comment on Genesis 1:29 (translated by Israel Abrams)
The strongest support for vegetarianism as a positive ideal anywhere in Torah literature is in the writings of Rabbi Abraham Isaac Hakohen Kook (1865-1935). Rav Kook was the first Chief Rabbi of pre-state Israel and a highly respected and beloved Jewish spiritual leader in the early 20th century. He was a mystical thinker, a prolific writer, and a great Torah scholar.
According to Rav Kook, because people had sunk to an extremely low level of spirituality (in the time of Noah), it was necessary that they be given an elevated image of themselves as compared to animals, and that they concentrate their efforts into first improving relationships between people. He felt that were people denied permission to eat meat, they might eat the flesh of human beings due to their inability to control their lust for flesh. He regarded the permission to slaughter animals for food as a “transitional tax” or temporary dispensation until a “brighter era” is reached when people would return to vegetarian diets. Perhaps to reinforce the idea that the ideal vegetarian time had not yet arrived, Rav Kook ate a symbolic small amount of chicken on the Sabbath day.
(Richard H. Schwartz, "Rav Kook and Vegetarianism" http://www.myjewishlearning.com/article/rav-kook-vegetarianism/)
“Ideally, humankind should be sustained by the produce of the earth. When, instead, other living creatures are used as food, as is permitted, such use should be restricted to living creatures that sustain themselves with what grows on the earth and that do not prey on other living creatures or attack man... Humans are unable to live up to the vegetarian ideal set forth at creation; God compromises and allows humanity to eat meat. But Israel wishes to adhere to that ideal, even in compromised fashion, and therefore Israel consumes only those animals that themselves have not killed other animals.” ” (Baruch A. Levine, Leviticus )
The Original Paleo Diet… “There can be no doubt in the mind of any intelligent, thinking person that when the Torah instructs humankind to dominate – "And have dominion over the fish of the sea, and over the birds of the sky, and over every living thing that moves upon the Earth" (Genesis 1:28) – it does not mean the domination of a harsh ruler, who afflicts his people and servants merely to fulfill his personal whim and desire, according to the crookedness of his heart. It is unthinkable that the Torah would impose such a decree of servitude, sealed for all eternity, upon the world of God, Who is "good to all, and His mercy is upon all His works" (Psalms 145:9), and Who declared, "The world shall be built upon kindness" (Psalms 89:3). Moreover, the Torah attests that all humanity once possessed this lofty moral level. Citing scriptural proofs, our Sages explain (Sanhedrin 57a) that Adam was not permitted to eat meat: "Behold, I have given you every tree... yielding seed for food" (Genesis 1:29). Eating meat was permitted to the children of Noah only after the Flood: "Like the green herb, I have given you everything" (Genesis 9:3). Is it conceivable that this moral excellence, which once existed as an inherent human characteristic, should be lost forever? Concerning these and similar matters, it states, "I shall bring knowledge from afar, and unto my Maker I shall ascribe righteousness" (Job 36:3). In the future, God shall cause us to make great spiritual strides, and thus extricate us from this complex question. (Rav Yitzchak HaCohen Kook Vision of Vegetarianism and Peace)
Animals During the Messianic Era… “At the end of days an inner thirst will prompt each person to search for someone upon whom to confer benevolence, upon whom to pour forth his overflowing spirit of kindness, but none will be found. For all humanity already will have attained happiness, living lives of delight, gratification, and prosperity in every sense – materially, ethically, and intellectually. Then, with all its store of wisdom, its collective insight and experience, humanity will turn toward its brothers on lower levels of Creation, the mute and the downtrodden, including the animal kingdom. And they will seek means to share wisdom with them, to instruct and enlighten them according to their abilities, thus to elevate them from level to level. There is no question that humanity will take an active part in this when the time comes to accomplish this mission. Beyond all doubt, humanity will share the enlightenment of the Torah with the animal kingdom, affecting their physical development and, all the more so, their ethical and spiritual development. This state of enlightenment will reach such a lofty level that we cannot imagine it at present, due to our lowliness and lack of wisdom. All beings shall receive a new, exalted form – a new world. [This is implied by the words of our sages:] "If they so desired, the tzaddikim could create a world" (Sanhedrin 65b).” (Rav Kook A Vision...)
The modern, mechanized, commercial animal factories that process most of the animal flesh we consume utilize methods that transgress the fundamental Jewish principle of “tsa’ar ba’alei chaim” - responding to the suffering of animals - and violate the spirit, if not the letter, of the kosher dietary laws. The contemporary commentator Rabbi Pinchas Peli, in Torah Today, writes that “the laws of kashrut come to teach us that a Jew’s first preference should be a vegetarian meal. If, however, one cannot control a craving for meat,...” the dietary laws should “...serve as a reminder that the animal being eaten is a creature of God, that the death of such a creature cannot be taken lightly, that hunting for sport is forbidden, that we cannot treat any living thing callously, and that we are responsible for what happens to other beings (human or animal) even if we did not personally come into contact with them.”
(Rabbi Jonathan Rubenstein, The Jewish World)