"Falling" - and Repentance and Redemption in LOTR

[Valandil]

This has been on my mind for a bit. I can think of four characters in LOTR who have a moral "fall" and yet have a chance of some sort of redemption, if they would repent:

1. Boromir - He falls by desiring the Ring to the point of attempting to lay hands on Frodo and take it. He repents of this to Aragorn and redeems himself by defending Pippin and Merry to his own death. Upon hearing of his fate, Gandalf seems more releaved that he repented before he died, than grieved that he DID die. His repentance appears to be quite important for his eternal fate... and yet, it strikes me as sad that he lived such a short span following his repentance.
2. Gollum - He of course fell long ago. Always inclined toward selfishness, he killed his friend and stole the Ring. He went on to become a thoroughly wretched character. Yet somehow, Gandalf held out hope that he would repent and have some sort of restoration or redemption through the choices he had yet to make. He even comes close - almost changing his mind about leading Frodo and Sam to Shelob. But ultimately, he fails to repent and 'dies in his sin' so to speak.
3. Saruman was a great being from The West. - He fell by lusting after power for himself. He betrayed the trust of those who sent him and he betrayed the friends he had made in Middle-earth. He consorted with Orcs. He desired to have The One Ring for himself and to become a Power. He used others to try and attain his personal ambitions. He personally betrayed Gandalf and attempted to destroy the Fellowship. And yet, Gandalf offered him repentance... first at Orthanc, then on the rode toward Rivendell. In his pride, Saruman refused to humble himself and admit that he had been wrong. Ultimately, his bodily form was detroyed, and his unrepentant spirit was rejected by the Lords of the West who had sent him.
4. Grima Wormtongue - He fell by betraying his own people, striving to attain for himself by subversive alliance a higher position than he could reach on his own merits - and to obtain for himself a particularly desireable young lady. Even when all was lost, he clung to Saruman, who had brought him to this. When their gambit failed, he became a haggard shadow even of what he had been before, and lowered himself to even baser vices. Even then, Frodo offered him another chance... to leave Saruman. But he could not - either he could not bring himself to leave him or his prior actions had made too great a barrier. He also died without peace - having just slain the only one who had given his miserable life any meaning.

One sad thing to me was that of these four who evidently had the option to repent, only one did so - and that one lost his life almost immediately after.

Curious to me - especially with Tolkien being a Christian, and repentance & redemption being the major theme of Christianity.

Each fell by desiring something for himSELF... something which was not otherwise their due, by birth, station, abililty or by all that was right.

Of course there are other creatures for whom repentance does not seem to be an option: Sauron, the Nazgul, the Balrog, Orcs - maybe even The Mouth of Sauron.

What do you folks think about this? Why did these four each have their chances to reverse themselves? Why did they, for the most part, reject that opportunity?