God created us for a reason. He wants us to to fulfill our function. Just as a carpenter builds a chair to sit on, and if it breaks as soon as anyone sits down on it, the chair is no good as a chair, and is only good as firewood. We must fulfill our function. A creation that doesn't work is useless to its creator, and is only fit for destruction.
God is a perfectionist. God wants us to be perfect. If we aren't perfect, we have failed to fulfill our function, and God has no use for us. The way for us to be perfect is to be obedient to God and to obey his laws. When we turn away from God and instead follow our own plans, we break God's laws. The punishment for breaking God's laws is death. This is not harsh. If we were to live forever in a state of imperfection and failure, we would become monsters. Imagine a once beautiful but now broken chair lying in in the mud. Now imagine that chair ten years later. Throwing it in the fire on the day that it breaks would be a mercy compared to letting it rot for years. So God decided that the punishment for breaking His laws is death.
When our first human ancestors turned away from God and broke His laws, God sentenced humanity to death. This was out of mercy, to limit our time on this earth, so that humans do not have infinite time in the freedom of the world to realize the full capacity of their evil.
God is merciful and loves us. So He wants to give us a second chance. He wants us to recover our perfection, despite our failure. The way to do that is for us to turn back to God, to have faith in Him, not ourselves or any creature, and to obey His law. But the punishment for sin is death, and we have already sinned. The inevitable consequence of sin is death. So how do we get a second chance? We have to kill. We have to spill blood. Blood is the price of sin, and so if we spill the blood of another in our place then the price can be paid without the cost of our own destruction. We have to willingly kill something precious to us, to show that we are sorry for our sin, to fulfill the punishment for that sin, and to show our resolution to return to full obedience to God, to prove our faith. This is sacrifice. In this way, by the grace of God, we can return to perfection and remove the reason for our own destruction.
The problem is, we can destroy our property, and we can kill our livestock, and we could even kill our own children or ourselves if such things were allowed, but we could never actually set our sin right, because all these sacrifices are imperfect. You would need some kind of perfect sacrifice to kill to perfectly restore us to perfection. We have nothing perfect to offer. So God stepped into the world, to die in our place, to serve as a sacrifice for our sins. Jesus incarnated. He offered to die in our place, a perfect sacrifice to restore us to perfection. We just have to have faith in the sacrifice that already proved to God that we are sorry for our sin, to fulfill the punishment for that sin, and to show our resolution to return to full obedience to God, to prove our faith.