The method by which the sin was to be covered was through the blood, the blood of animals (an animal without blemish - the best one), was called the sacrificial lamb. Why the blood? ....because blood is the life of all things, animal and human. Without blood there is no life. When sin is committed there is death to the life of that person in a spiritual sense. The death is the punishment due to sin. If God demanded death every time the Jews sinned - there would be no Jew left - for all men sin - we have a sin nature from Adam. So, in God's love, He required the sacrifice of animal blood instead of human blood to cover the sin. When the animal's blood was spilled, sin was covered and God could then see the blood of life - not the sin. He would then forgive the Sinner. This Law of Sacrifice was to continue until the promised Messiah would come and replace the animal sacrifice. Christ would then become the blood sacrifice for all mankind, for all their sins, forever (1 John 1:29-36, 1 Peter 1:19, Rev. 5:6-13). God, knowing that man could not even keep the Law and would still sin, as we do today, made it easy to be forgiven, He sent His Son to be our sin cover. We place our sins under the blood of Christ. God now sees His Son's blood and not our sins, when we repent of them. He made salvation easy under Christ.
Christ had to die so that he could rise again in full human form, the same body he died in, physically alive again. By doing this he defeats the curse of death, rising and living eternally, as origianlly intended for humankind. We, with forgiveness and salvation, receive the same from Christ as he raises us into eternal life after of earthluy death. Because of Christ Jesus, we also can defeat the curse of death.