This can be a difficult verse and could mean different things to different people obviously. I interpret it like this: Sin has always been in this world, or better said, exists because Satan is ruler of this world right now. However how is one to know "sin" or "right from wrong" if there are no guidelines. If we never knew the law we could sin forever and never know it was wrong. But because the law was given our sin was brought to light. Now we have no excuse. We have been made aware of right and wrong. The law was indeed given to magnify our sinfulness and help us realize we are in need of a Savior.