In heaven, we can be with God face to face. There we can be free from sin and its power and effects. Hell is separation from God. It is sin that takes man to hell. It is by sin that man does his evil acts.
Sin requires a consequence.
When Adam sinned, he was separated from the Garden of Eden, the beautiful place He had lived and met with God. Also, as a consequence to his sin, God told him that he would die. And he did eventually die. To postpone the death, and to make it so that God did not have to be totally separated from Adam, the Father told him that He would require a life - a blood sacrifice. So Adam sacrificed an animal.