In Exodus 12, when the last plague came and the angel of death went through Egypt killing the firstborn, the Hebrews had been instructed by God to kill a lamb in sacrifice and put its blood on the doorposts of their dwellings. God's wrath would pass over the people inside who had partaken of it and were thus under the blood. The Bible teaches that blood represents life. Death is the penalty for sinning. To sin is to break God's law.
Sin is an intruder in the earth.
It is not natural. It is not necessary. In fact, the human race is 6000 years behind what it could be exactly because of sin. The devil is the initiator of sin. We know that he is in direct opposition to God.