In fact, the law requires that nearly everything be cleansed with blood, and without the shedding of blood there is no forgiveness. Hebrews 4:22

If you are arrested for drunk driving, the state may say the wages or payment you must make for drunk driving is one year in prison. The legislature determines a fair punishment equal to the crime. It is not fair to execute someone for jaywalking.

Paul in the book of Romans says the wages or payment for sin is death. Something must die to pay for the damage of sin. It all began in Genesis. When Adam and Eve sinned, God (in chapter 3) said man will now return to the dust of the ground that they came from. Death was the penalty/the sentence for sin.

God allowed something else to die for our sins during the Passover (Exodus 12) and by outlining the sacrificial system using sheep and goats (Leviticus 4, 5). The sinner put his hands on the sacrifice, confessed his sins, then the animal was killed. The sins were put on the animal and the animal took on the sins and died for the person.

Once the payment was made, God was satisfied and the sins were forgiven.

The spilling of blood indicates the death of something, in this case the sacrifice. So when a sacrifice dies, it takes the place of something that should have died for their own sins. Sin must be an incredible offense to God to require such a drastic payment and God wanted to communicate that by making this sacrifice a terrible moment to witness.