Oscar Pistorius -- Called a 'Reckless' Disaster Waiting to Happen by Fellow Paralympian

Oscar Pistorius was a ticking time bomb and a disgrace to Paralympians everywhere -- even before he was charged with murdering his girlfriend -- this according to a U.S. Paralympic gold medalist who says he's "not surprised" by the tragedy.

Todd Schaffhauser tells TMZ he "warned many of the prosthetic sponsors about Oscar after reading some of the first incidents of his reckless behavior" on and off the track.

Todd -- who won gold as a sprinter in the '88 Paralympics (below) -- was referring to allegations Pistorius wrecked a speedboat, fell asleep at the wheel and crashed a car ... and also to the fact he was arrested for allegedly assaulting a different girlfriend in '09.

Although Todd's never met Pistorius -- he adds, "Oscar truly showed the only disabled person he was looking out for was Oscar" ... when he lost the gold medal at the London Paralympics, and accused the winner of cheating.

"I never saw him as a true ambassador of the sport. True ambassadors don't behave as he has in the past.”