This seems to be a minority opinion, but I think Hal Jordan becoming Parallax (for real, not because he was possessed by a "fear demon" or whatever dumb bullshit Geoff Johns came up with to hand-wave it away) and then sacrificing himself in a demonstrative way in FINAL NIGHT was, in the main, a pretty logical and organic development for the character.
In THE COMIC BOOK HEROES, Jacobs and Jones (and yes, I know about Jones, don't @ me) describe Hal Jordan as the ultimate "organization man," and that's stuck with me because I think it identifies an important feature of the character. Hal first appeared in 1959, but he was the quintessential Kennedy-era hero, a charismatic individualist who drew strength from the conviction that he was part of a just liberal order in a rational, knowable universe.
This was true even in his early appearances, when he had a very limited awareness of the Green Lantern Corps and no idea who they were working for (which is something that has, regrettably, been retconned from latter retellings, although it was an important part of the earliest issues of the series). What made Hal so successful as a Green Lantern was not that he was especially clever, imaginative, or even strong-willed β it was that he respected the parameters of the role, even when no one was looking, and worked creatively within the rules by which he was bound. If he were more intellectual, he would have been a great lawyer.
The side effect of those tendencies was that, like many liberals, Hal could not cope gracefully with challenges to his world view, and his response to such challenges was authoritarianism. Denny O'Neil called Hal "a crypto-fascist," and that was evident in the 'Hard-Traveling Heroes" stories with Ollie, where Hal faced a series of crises challenging his deeply held presumption that laws and institutions are basically good. His reaction to those crises was panic and rage β lashing out in an attempt to reestablish his own rules of moral order through force, as authoritarians do.
That was the driving force of his disintegration in "Emerald Twilight," where his reaction to being told that he couldn't just use his power to forcibly return things to the way he thinks they should be was an explosion of power-hungry rage. That escalated more quickly than I think felt credible, especially in Hal's sudden willingness to kill his own comrades, but the gist was consistent with his previous trajectory, as was his goal in ZERO HOUR, to go back to the Dawn of Time and rewrite the rules as he thought they should be.
What gave ZERO HOUR's climax its (admittedly very limited) emotional weight was that Hal still saw himself as a right-minded reformer who was still ultimately serving the cause of moral order. That I think could have been Hal's most significant legacy as a character: an actual cautionary tale, not confined to an Elseworlds Annual or Imaginary Story, about someone who used to be a hero until he went too far. (Often, in modern Elseworlds-type AUs, that narrative centers on Superman, but Hal was, and remains, a more credible choice because it didn't require making him into anything he wasn't already.)
The finale of FINAL NIGHT then gave Hal a grandstanding opportunity for redemption: He sacrificed his life and his remaining power to destroy the Sun-Eater and save the world one more time. Probably the most interesting aspect of that story was that some of his former comrades β notably Batman β were not sold, recognizing that it was really just Hal continuing along the same path that had been his moral downfall in the first place, and it didn't make up for the things he'd done or the people he'd killed.
I can see how for a Silver Age Green Lantern fan, that was something of a bitter pill, but I *don't* think there was a way back for Hal from being Parallax, and so ending on an ambivalent note was most appropriate. If it had been up to me, they would have left it there.
