what does this quote mean

Question

Prisons are full of people that were simply being themselves.

Answer 1

It seems fairly straightforward. 

 

This quote claims that most people in prison were merely following their own natural impulses with no regard for societal rules.  Thus, their natural inclination was to commit whatever crime got them incarcerated.

 

This is the reason society has rules and laws.  Those who just "be themselves" with no regard for those rules and laws end up in jail (if caught).