There's no doubt that if Green Lantern knew a secret from Sinestro's past, he'd absolutely go ballistic. Thanks to the interference of Booster Gold, Hal Jordan's greatest enemy was inspired to do his most villainous undertaking ever.
When Hal Jordan became the first person from Earth to the illustrious Green Lantern Corps, he faced a number of naysayers, including his future rival, Sinestro. However, Hal proved himself to be not only a competent ring-slinger, but showed the potential to be the greatest Green Lantern ever. The seasoned officer came around to Jordan and mentored him, until Sinestro betrayed the Green Lanterns, cementing their rivalry for years. Since Sinestro's fall from grace, Hal has taken the actions of his former teacher personally and done everything he can to keep Sinestro in check. But Sinestro's dedication to bringing the universe to order through fear makes him one of Jordan's most recurring headaches.
And if Hal ever discovered who helped Sinestro on his path to villainy, his head might just explode. In Booster Gold #2 by Geoff Johns, Jeff Katz and Dan Jurgens, the titular hero is informed that due to a temporal interference, someone has sent Sinestro after a pre-Green Lantern Guy Gardner. Though Sinestro is still an active Green Lantern, Booster is told that if Sinestro meets Guy years before he's intended to it will lead to Guy's death. Booster goes about with his mission and is able to prevent Sinestro from crossing paths with Gardner. By presenting himself as a fan of Sinestro's from the future, he distracts Sinestro and prevents him from meeting with Gardner. As Booster departs, Sinestro notices Gold's Legion Flight ring and asks what kind of corps uses a yellow ring, causing Booster to reply with "the...Sinestro Corps," greatly pleasing the future villain.
While Booster Gold might seem like a joke hero, he's actually one of the most dedicated protectors of timeline. Though the scene here is played for laughs, this is Booster doing what he does best; ensuring that events happen exactly the way that they are supposed to. Still, just because Booster is doing the right thing, that doesn't mean that Hal Jordan would be able to swallow the truth any easier.
Hal Jordan has a long, complicated history with Sinestro. Not only did he feel betrayed at the actions of his mentor, the Sinestro Corps has taken the lives of a number of Green Lantern Corps . Needless to say, Sinestro’s development as a villain is a sore point for Jordan. If he discovered that Booster Gold, someone who's been a member of the Justice League, played a part in putting Sinestro down his dark path, he would most certainly not be happy. Booster is used to people being angry or thinking poorly of him, so one more among the numbers wouldn't change much. But for Green Lantern, he'd practically have to restrain himself from hurting Booster knowing he helped make Sinestro the villain he is today.
