The straightforward answer to the question is this: sin entered the world through one man, and death through sin. (Romans 5:12)
The first man and woman (Adam and Eve) were given great freedom, but also one serious warning. You must not eat from the tree of the knowledge of good and evil, for when you eat of it you will surely die. (Genesis 2:17) This was an ideal test of man's willingness to obey what God said simply because God said it. But the devil tempted Eve to disbelieve and disobey God's words, and she did. When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it. (Genesis 3:6)
At that moment 'sin entered the world'. By his deliberate disobedience man cut himself off from God. Instead of loving God, Adam and Eve were terrified of Him: they hid from the Lord God among the trees of the garden. (Genesis 3:8) Instead of being assured confident and happy, their sin had made them ashamed, guilty and afraid.
But God had said that man would die if he disobeyed, and he did. Death means separation, and in one terrible moment man became separated from God; he died spiritually. He also began to die physically, and now had a dead soul and a dying body. But that was not all: the children of Adam and Eve inherited their corrupt nature and sinful character. From then on, like pollution at the source of a river, the poison of sin has flowed to all Adam's descendants, and in this way death came to all men, because all sinned. (Romans 5:12)
Notice that important word 'all', which obviously includes the writer and the reader of this page. We may never meet on this earth, but we have this in common — we are sinners and we are dying. If we claim to be without sin we deceive ourselves and the truth is not in us, (1 John 1:8) and if we claim not to be dying we are being ridiculous. Fooling around with the facts does nothing to change them.
Many of today's newspaper, television and radio headlines remind us of the fact that the world is in a mess. It is easy to condemn violence, injustice, disorder and wrongdoing in society, but before criticising others ask yourself whether you are perfect and living a life pleasing to a holy God. Are you absolutely honest, pure, loving and selfless? God knows the answers to these questions — and so do you! All have sinned and fall short of the glory of God. (Romans 3:23) You are a sinner by birth, by nature, by practice and by choice, and you urgently need to face the facts — and the consequences.