It is odd that the Guardians of the Universe are not held responsible in any real way for their role in why things went bad for the Corps. If they weren't so full of themselves, they would have seen a long time ago how much Sinestro hates them. He barely even attempts to hide how much he despises how the Guardians have turned the GL Corps into trash collectors. Also, even before Hal Jordan had a chance to say much of anything to them, the Guardians were already insulting him. Jordan defends himself, and the only character who initially respects Jordan at all is Sinestro. That's right, of all the Green Lanterns or their leaders, Sinestro is the first one to see Hal Jordan's immense potential. Sinestro was quite selfish, but he was smart enough to suspect Jordan could be the only one capable of thwarting what he was planning. That, I believe, was why he orchestrated the Guardians taking Jordan's green ring and then offering him a yellow one: he knew Jordan was possibly the only one with a clue. Before Sinestro crossed the Moral Event Horizon he had respected Jordan better than anyone and the Guardians only respected Jordan after he'd prevented the complete collapse of the Corps.
The Guardians are flawed people, like everyone else. Just because they don't suffer for it doesn't mean they're not wrong.
The Guardians being pretty terrible is a fairly consistent portrayal in other media too.
Honestly, this troper found this movie to be one of the few actually benevolent portrayals of the Guardians in recent years
The Guardians can read and probe onto people's minds. Why do they bother with courts martial when they have a way to see beyond any doubt. Is it a privacy thing?
The Guardians desire order as much as Sinestro, if not more. The problem is that they often have Lawful Stupid plastered on their heads. Following due process when there's an easier alternative would be just like them.