Within every heart is a darkness in which every mankind was born with. However, when the darkness in the heart becomes too strong, an inner demon forms within the bearer of that heart. This was known as the "Heart Of Sin" because each crime committed makes the darkness within the person grow strong and thus, a mysterious phantom appears to strike down the inner demon but also mentally harm the person during the process. This goes to show you that each crime committed is a crime against humanity and crime against humanity is evil towards others. When you hurt others, others are bound to do evil against you.