Genesis 1:29 says, “And God said, “Behold, I have given you every plant yielding seed that is on the face of all the earth, and every tree with seed in its fruit. You shall have them for food. And to every beast of the earth and to every bird of the heavens and to everything that creeps on the earth, everything that has the breath of life, I have given every green plant for food.” And it was so.” So we know that in the garden of Eden God only intended us to eat plants for food, not animals. I guess the question is, what changed?
By Genesis 9:3, we see written that “Every moving thing that lives shall be food for you. And as I gave you the green plants, I give you everything.” So we know something had to happen in the first ten chapters of Genesis which would cause God to change the rules for Man and allow us to eat meat.
One clue could be found in Genesis 6:7 where “(So) the Lord said, “I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them.” Here “them” includes animals. Why is God sorry he made animals?
If there is an answer in the bible, it can only be in a place where animals are mentioned in early Genesis.
Genesis 6:13 says that in some way, animal life had become corrupted along with human life. It says, ““I have determined to make an end of all flesh, for the earth is filled with violence through them.”” The phrase “all flesh” is used throughout the narrative to include both human and animal life. How was animal life corrupted? This is not explained directly, but in other parts of the bible we see that a person’s sin can contaminate other people or animals at times (e.g., Joshua 7:24-25; Romans 8:19-22). One famous example would be Rachel stealing her father's idols, which although it was done to help her father is said to have eventually caused her death. Another example would be Genesis 13 where we see “14 The Lord said to Abram, after Lot had separated from him, …” which shows that God was waiting for Lot to leave Abraham's presence.
The sages give some explanations on this as well by pointing out two odd statements in Genesis 6. In Genesis 6:2 it is written that when the Sons of God saw the daughters of men, they chose from among anything they saw fit. The explanation is that since they didn't merely chose from among the daughters of men, they may have engaged in sin with animals, corrupting them. They also point out in Genesis 6:13 where God does not only command Noah to take animals two by two, and also specifically commands Noah to take them as male and female. The implications here is that the animals were both cohabiting with different species, and that they were failing to reproduce male and female. In other words, that the natural order had somehow become corrupted. This plays into the earlier statement that men (as well as animals, as implied by 'all flesh') had become corrupted, and how, by choosing from among anyone or anything they saw fit.