Hertz malfeasance

We hired a car from El Pratt airport Barcelona. The car was returned in perfect condition with a full tank. Hertz charged an additional €500 to my credit card and on being challenged about it, they are clearly "ducking and diving" If you take the time to look at their reviews online, the ***** and malfeasance is rife. How is it possible that a huge company like Hertz has become a such cess pool. In principle, I refuse to give up on this! Hellopeter, and any other good Samaritans please help!