Was it on the basis of the devastation of the flood that man was allowed animal flesh?
I think not. The reason, I believe, is found from the wickedness of the earth in the time before the flood.
After God created man, He gave them authority over the fish in the sea, and the birds in the sky and over every living creature that moves on the ground.
From Genesis 1:
28 God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish in the sea and the birds in the sky and over every living creature that moves on the ground.”
God then told Adam and woman:
29 Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food.
And this extended to all of the air breathing creatures:
30 And to all the beasts of the earth and all the birds in the sky and all the creatures that move along the ground—everything that has the breath of life in it—I give every green plant for food.” And it was so.
In the beginning, nothing ate meat!
I'll start with Genesis 4.
The story of Cain and Able is interesting. Cain worked the soil, and Able tended his flocks.
Soil is described throughout Scripture as something that man 'has that he can develop', and 'tending flocks' describes a relationship of one who actively works to ensure the health and welfare of those he cares for.
Genesis 4 states:
2 . . .Now Abel kept flocks, and Cain worked the soil. 3 In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. 4 And Abel also brought an offering—fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, 5 but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
As God was not pleased with Cains offering, "working the soil" must describe how Cain interacted with the lives of those whom he had control over. Beating others to force compliance, lying, cheating, stealing, and killing, all describe a few harsh methods of "working the soil". The results of Cains harsh and ugly treatment of those he had control over would have been his offering to God.
Able, on the other hand, tended flocks. Tending flocks indicates a relationship of one who nurtures and cares for others. And because Able's offering pleased God, Ables relationship with his family and others would have been a loving one.
Psalm 23 states:
1 The Lord is my shepherd, I lack nothing. 2 He makes me lie down in green pastures, he leads me beside quiet waters, 3 he refreshes my soul.
He guides me along the right paths for his name’s sake. 4 Even though I walk through the darkest valley, I will fear no evil, for you are with me;
your rod and your staff, they comfort me.
Able was a shepherd of people, and Cain was a user of people. Cain was resentful that God was pleased with his brother and not him, so he killed Able and felt no remorse.
Perhaps God blessed Able, and not Cain, with prosperity. And perhaps Cain resented his brother because of his wealth.
The people at that time would have been genetically perfect. Disease would have been non-existent. People at that time lived incredibly long lives and the females would have had many, many children. All females born before the flood would have been taken by any man at any time unless they were protected by a strong family.
It appears from the verses of Genesis 3 that man could speak with the animals. As man had direct control over the animals, the animals would do mans bidding. Over the course of the years before the flood, even the animals became wicked.
Genesis 6 states:
5 The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. 6 The Lord regretted that he had made human beings on the earth, and his heart was deeply troubled. 7 So the Lord said, “I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them. 8 But Noah found favor in the eyes of the Lord.”
By the time of the flood of Noah, man and the animals had progressed from eating from the seed-bearing plants on the face of the earth and every tree that had fruit with seed in it, and every green plant, to eating meat. Even Noah and his family ate meat.