Let's assume that there are two warring factions, Red and Blue.
In the course of events, a member of blue faction gouges out the eye of a member of Red faction.
In the spirit of "An eye for an eye" (a concept which both factions strictly adhere to) another member of Red faction takes the eye of the offending member of Blue faction.
The trend of vengeance then continues, as I will illustrate step-by-step with the following unnecessarily long sequence of pictures:
The cycle of violence continues until all people have been blinded except for one man with one eye left, leaving him to rule over the blind masses like a god.
So 'an eye for an eye' doesn't leave the whole world blind, it leaves a single one-eyed man ruling billions of blind, helpless minions. Hopefully that helped dispel some of the myth.