At the end of Gen 3, God clothes Adam and Eve in coats of skin, replacing the garments of leaves they'd sewn for themselves in order to cover their shame. Day in and day out, as they wore the coats of skin, what did they think prompted God to clothe them so? Did they miss the fact that skinning an animal results in the death of the animal? Did they fail to realize that the covering God provided required the death of another?
Gen 4 begins with the account of Cain, Abel, and their sacrifices. As Substitute mentioned, ritual is symbolic representation of theology. What is the theological significance behind killing an animal, and how does it differ from killing a plant? Is it fair to say that Abel's sacrifice is a symbol of the death of another, but Cain's sacrifice isn't? I think so. As Jennifer suggested, the spilling of blood is essential, for it's a clear symbol of the terrible nature of death. Abel's sacrifice shows that he understood that justification is made possible by the death of another, and Cain's sacrifice shows that he didn't understand this.