"Albo’s theory took as its starting point, not sacrifices but two other intriguing questions. The first: Why, after the flood, did G-d permit human beings to eat meat? (Gen. 9: 3-5). Initially, neither human beings nor animals had been meat-eaters (Gen. 1: 29-30). What caused G-d, as it were, to change His mind? The second: What was wrong with the first act of sacrifice -- Cain’s offering of “some of the fruits of the soil” (Gen. 4:3-5). G-d’s rejection of that offering led directly to the first murder, when Cain killed Abel. What was at stake in the difference between Cain and Abel as to how to bring a gift to G-d?

Albo’s theory is this. Killing animals for food is inherently wrong. It involves taking the life of a sentient being to satisfy our needs. Cain knew this. He believed there was a strong kinship between man and the animals. That is why he offered, not an animal sacrifice, but a vegetable one (his error, according to Albo, is that he should have brought fruit, not vegetables – the highest, not the lowest, of non-meat produce). Abel, by contrast, believed that there was a qualitative difference between man and the animals. Had G-d not told the first humans: “Rule over the fish of the sea and the birds of the air and over every living creature that moves in the ground”? That is why he brought an animal sacrifice. Once Cain saw that Abel’s sacrifice had been accepted while his own was not, he reasoned thus. If G-d (who forbids us to kill animals for food) permits and even favours killing an animal as a sacrifice, and if (as Cain believed) there is no ultimate difference between human beings and animals, then I shall offer the very highest living being as a sacrifice to G-d, namely my brother Abel. Cain killed Abel not out of envy or animosity but as a human sacrifice.

That is why G-d permitted meat-eating after the flood. Before the flood, the world had been “filled with violence”. Perhaps violence is an inherent part of human nature. If there were to be a humanity at all, G-d would have to lower his demands of mankind. Let them kill animals, He said, rather than kill human beings – the one form of life that is not only G-d’s creation but also G-d’s image."

http://www.chiefrabbi.org/thoughts/tzav5767.html