Why was a 'blood sacrifice' required to to atone for the sins of mankind?

In the Bronze Age blood was an important symbol to people. It was the life force that came from the heart. If you wanted to prove you were sincere, you made an oath in blood. If you wanted to atone, you made a sacrifice in blood. It is easy to look back 3,000 years and say "how barbaric" but it's not fair. It's like making fun of Shakespeare because he used funny words while you miss the value of what he wrote.