For this you have to look at guys like Scott Hall or Jake Roberts
Both very talented men who have ruined their own careers through drink and drugs. It is one thing to abuse steroids (I do not agree with it, but people do it in an attempt to BOOST their careers), but these 2 prevented themselves from reaching their undoubted potential through alcohol and druge abuse. I have no sympathy for either Hall or Roberts, they are simply wasters who had all the talent but were just too far gone to recover.
On a slightly different note I would like to mention Teddy Hart. The man is an incredible high-flyer, part of the Hart family (which opened doors for him), and was the youngest guy ever to be signed to a WWE contract. However, Teddy has been fired and rehired on numerous occasions without ever making it to the main roster as he causes disruption wherever he goes, with his attitude, backstage antics and rubbing up wrestlers the wrong way, often going into business for himself in matches and no-selling finishing moves.
Hart has the opportunities, every advantage in the book and made absolutely none of it. He has never been a part of the roster of a major wrestling company, has a bad reputation throughout the business, many enemies and it is all his own doing.
Also, if we are talking about people who have destroyed their legacies, then there is only one who stands out. That man is Chris Benoit