If you will turn to Lev xvii, you will find why God demands blood. "For the life of the flesh is in the blood, and I have given it to you upon the altar to make an atonement for your souls; for it is the blood that maketh an atonement for your soul."
Suppose there was a law that man should not steal, but no penalty was attached to stealing; some man would have my pocketbook before dinner. If I threatened to have him arrested, he would snap his fingers in my face. He would not fear the law if there was no penalty. It is not the law that people are afraid of; it is the penalty attached.
Do you suppose God has made a law without a penalty? What an absurd thing it would be! Now, the penalty for sin is death; "The soul that sins shall die." I must die or get somebody to die for me. If the Bible doesn't teach that, it doesn't teach anything.
And that is where the atonement of Jesus Christ comes in.
D.L. Moody, page 28 in One Thousand Evangelistic Illustrations, edited by Webb, A. (1924). New York: Harper & Brothers Publishers