Makes all the sense in the world, for Sinestro.
(I'm stealing from a reviewer)
Not the Sinestro as we see him in this film. He's the best character in a goodness sense. Internal logic doesn't support him donning the ring. Some hints needed to be dropped about a character flaw, or even showing him feeling seduced by it a la Bilbo.
IIRC, you just see him putting it on. Why? So he can become evil for the sequel, is all. I liked the move in general for its not-so-pretentiousness, but the Sinestro-ring thing did need a touch of characte develpment to explain it, IMO.
Agreed 100% I'm gonna get into a little bit of fan-wank territory here, but I have an idea for what they really should have done. One, sad to say it as he was probably the best thing in the film, but ditch Hector Hammond. Replace him with Atrocitus, and have Atrocitus be the one to kill Abin Sur. Then Sinestro takes Hal under his wing as the two of them track down Atrocitus together. This gives plenty of opportunity to show that Sinestro is perhaps not entirely stable. Have him be brutal and ruthless in his pursuit. Then once they've caught up with Atrocitus, he give Sinestro a peek at his future, causing Sinestro to murder him (Not my choice, but there seems to be a rule that the bad guy has to die in superhero movies.) If they were really dead-set on Parralax, they could have had Atrocitus as a minion of him. He laughs as he dies and warns them that something much bigger than him is coming. It could be made to work. Perhaps Atrocitus turned to Parralax in his quest for revenge on the Guardians.
There you go. That would have made the film far less Earth-bound (a primary complaint,) given Sinestro much more screen time and development while making it far more believable that he would become a villain (another complaint,) and working in Atrocitus, who is a fan favorite. Also, if they're pursuing Atrocitus across the galaxy, it allows them to work in more Lanterns, with brief team-ups. Boodika, Arisia? Come one. give the Corps more to do.
So yeah, there's my totally pointless "how I would have made Green Lantern" rant. Of course, that's not the movie they made, and it will never be made so.....yeah.