Heroic self-sacrifice is rarely seen in peacetime, and when it happens it becomes a media event.  We love to read a story of bravery.  Paul's argument about the sacrifice of Jesus Christ is different.  The death of Jesus was no accident or regrettable consequence of saving another's life: it was planned.  Jesus clearly understood that His life's work was to die (Matthew 16:21-23).

How did death come into God’s perfect creation (Genesis 1:31)?  It came because of sin.  The first act of rebellion against God started by Satan sowing doubts into Eve’s mind (James 1:13-15).  Doubts about God’s goodness were followed by the devil deliberately contradicting God’s Word - before presenting an attractive reason why the divine command meant nothing, and human feelings meant everything.  Adam was with Eve listening to Satan, and they agreed to defy God (Genesis 3:1-6).  But when they did, the sweet fruit immediately turned sour in their hearts, and they wanted to hide from God:

Jewish law demanded 'an eye for an eye and a tooth for a tooth' (Exodus 21:24).  Personal injury to other people must be paid for personally - the punishment would be the same as the injury caused (Leviticus 24:17-22).  One person would pay for each person who suffered.  

Some people think that the definition of sin is "breaking a law".  But no!  Adam, Noah and Abraham did not have the law of Moses, but they were still sinners.  Sin is rebellion and disobedience to the Word of God. The Old Testament law was added later.  It does not create sinfulness but defines the offence so that an appropriate punishment can be prescribed.  So then, the law makes sin more obvious with a known punishment which is to be feared.