Leviticus 4:27-31 (NKJV)
'If anyone of the common people sins unintentionally by doing something against any of the commandments of the Lord in anything which ought not to be done, and is guilty, or if his sin which he has committed comes to his knowledge, then he shall bring as his offering a kid of the goats, a female without blemish, for his sin which he has committed. And he shall lay his hand on the head of the sin offering, and kill the sin offering at the place of the burnt offering. Then the priest shall take some of its blood with his finger, put it on the horns of the altar of burnt offering, and pour all the remaining blood at the base of the altar. He shall remove all its fat, as fat is removed from the sacrifice of the peace offering; and the priest shall burn it on the altar for a sweet aroma to the Lord . So the priest shall make atonement for him, and it shall be forgiven him.
This passage talks about an unintentional sin for which we are still culpable. To our natural minds this seems unfair. We did something wrong and didn't know it? In God's economy, we still must be redeemed and forgiven of this guilt. Willful disobedience will never be forgiven. But this speaks of our sin nature. Once we become aware of it, we must carefully take the steps prescribed by God to atone for it.
The Bible speaks of a very bloody religion. Such a requirement by God seems horrific. God wants us to understand his absolute holiness, our absolute lack of holiness, and the unfathomably costly sacrifice He has made to allow us to bridge the gap.
The Bible tells us that the rituals of the Old Testament were mere "shadows" of the real to come. The animal had to be without blemish, just as Jesus was perfect. The animal was a substitute, just as Jesus is our substitute. Each sin was placed on the animal identifying it with the sin, just as Jesus identified with us and our sins taking them on himself. The animal had to die giving its life to the offerer, so Jesus had to die giving us access to His life.
Hebrews 9:22 (NKJV)
And according to the law almost all things are purified with blood, and without shedding of blood there is no remission.