No, I'm talking about the animal sacrifices. From my understanding of Genesis, before Adam and Eve's disobedience, neither the humans nor the animals wanted to kill one another. Since they all lived according to God's Will before the fall, I assume that it was God' Will for humans and animals to not kill one another.
So what I don't understand is what exactly made God approve of humans killing animals and bringing their corpses to Him as a way of honoring Him. Did Abel come up with this idea on his own or did he learn it from his parents or from someone else? And whoever the first human that got the idea of killing animals was, why wasn't this human told by God that killing animals causes them suffering and is against God's Will?
Cain bringing grains to God is a bit more logical in the sense that they ate grains, so it was something along the lines of: "when us, humans, share food with each other, we enjoy our presence more, so maybe if I share food with The Being Who created me it will make This Being enjoy my presence more." So he brought grains, the food that he grew from the ground, to God. Granted, The Bible doesn't say that it was the best grains he had, so I understand why God would not regard Cain's offering. But the idea of bringing food to God is still grounded in logic, when compared to the idea of killing animals (which is basically destroying God's creation, bringing its remains to God, and telling God that you destroyed His creation as a means of honoring Him).