Imagine No Redemption

[Pitchwife]

Interesting thread and discussion, Hookbill & company!

I think Noggins has the right of it above and "Tolkien's particular view of catholicism" has something to do with all this. It was Tolkien's belief that not only we but the world with us are Fallen from grace, a belief which is represented in his legendarium by the concept of Arda Marred (which Legate already mentioned) or "Arda with a Morgoth-ingredient". In such a world, turning from good to bad is always easier than the reverse, because it means moving into the same direction as the tendency of the world itself, whereas turning from bad to good means you have to struggle against the current, so to speak (which, I suppose, is why, according to catholic faith, we're unable to redeem ourselves but need to be redeemed by Christ, which hasn't happened yet in Middle-earth).

Concerning people like Boromir, Isildur and Thorin dying after repenting, I'd suppose that death could be seen as an atonement for their sins, but I'd also suggest a narrative reason for Tolkien to kill them off. Like either Bilbo himself or the translator of his memoirs reflects in The Hobbit:

Now that is a strange thing, but things that are good to have and days that are good to spend are soon told about, and not much to listen to; while things that are uncomfortable, palpitating, and even gruesome, may make a good tale , and take a deal of telling anyway.

But not only that - I'd say things (and people) changing from one to the other are always more interesting to tell and read than things remaining the same. A character who turns from good to evil and back has exhausted their narrative potential - you can hardly write anything more interesting about them to top that, so they're best written out of the story; and in an adventure story set in a heroic age, how better to accomplish this than by having them slain. (If I may adduce a comparison, George Lucas knew perfectly well that we didn't want to see Father & Son Skywalker reunited re-establishing the Republic happily ever after; what we cared for was the moment when Anakin's love for this son overcame the hold of the dark side on his soul, at the price of his life.)