"Seek no further, O man, to solve the dark riddle of sin;
Suffice it, that thine own bad heart is to thee thine origin of evil." Martin F. Tupper.
We all wrestle with the origins of sin and why there is such evil in the world. I've never heard nor thought of a satisfactory answer and I'm reasonably certain that this side of heaven there will be no solution. I think this quote sums it up as well as can be.