The story of Cain and Abel is simple. Cain trusted in his human ability, the work of his hands for God. Abel trusted and leaned on God's ability. Able trusted fully in the work of God. Cain, having been shown the right way by God he was unrepentant. He was interested in what he could do for God and God better be impressed by his work whether he liked it or not. If God didn't like it, well then, if He wants blood He would see it, so he spilt the blood of Abel. What arrogance? (A shadow of the betrayal that would save us all in Judas, pharasees spilling the blood of Christ) .
Abel knew God was Able so he gave a sacrifice to God that simply showed that he was RECEIVING God's salvation, just as we receive the work of Jesus as the Lamb sacrificed. He thus acknowledged God as the One who was able to save.
Thus, the sacrifice of bulls and goats was never something that could take away sins but a ritual that pointed to the real sacrifice off Christ that whoever looks to Christ shall indeed be saved. The sacrifice of bulls and goats and lambs was thus always about saying Lord we look to You as the One who saves, as the One who takes away the sin of the world. We look to the real Lamb to come who takes away the sin once and for all.
The sacrifice was thus always about salvation and the Mesaiah. Cain said no thanks to Your Saviour, I won't just kill a lamb i'll kill the man You are pleased with because he is casting a bad light on my human works.
Abel comes in humility declaring the coming of the Saviour. We come humbly receiving God's finished work of salvation that does not need our work. We say yes to the work of God. Yes! We look to the coming King Groom. We say"come Lord Jesus, come"