Is man good?
The bible tells us that God is good and man is bad. Man has a sinful nature.
Why does sin exist?
God created man without sin to have fellowship with him. But man disobeyed the commandment of God:
And the LORD God commanded the man, saying, "Of every tree of the garden you may freely eat; but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die." (Genesis 2:16-17, NKJV)
Adam and Eve ate the fruit of the forbidden tree and sin entered them and spread to all men:
Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned (Romans 5:12, NKJV).
Nobody can escape death: "What man can live and not see death? Can he deliver his life from the power of the grave?" (Psalm 89:48, NKJV)
All men are accountable for their own sin. "So then each of us shall give account of himself to God." (Romans 14:12, NKJV).
Sin is punished by death and eternal separation from God: "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord." (Romans 6:23, NKJV).
Jesus delivers us from sin and gives us eternal life.
Jesus paid the ransom
Jesus saves us from sin and death. Jesus gave himself up as a sacrifice for all mankind to redeem them. Men are held hostage by sin and the devil; Jesus paid the ransom to give them back to God, their heavenly Father. "For even the Son of Man did not come to be served, but to serve, and to give His life a ransom for many." (Mark 10:45, NKJV)