Like the sibling comment remarks, Tolkien never fully embarked on this path.

He had a problem: as a Catholic [1], he thought every creature deserved pity and second chances (you can see this when Gandalf rebukes Frodo when he says "it's a pity Bilbo didn't kill Gollum"). If the Orcs are really "fallen Elves", they deserve pity and maybe mercy; they are worthy of redemption. Yet Orcs in LotR are to be killed on sight; there's only one passage in all of LotR where the Hobbits reflect on the corpse of an Orc with any kind of attempt at insight.

For Orcs to be a thing to be destroyed without mercy, unworthy of redemption, they must have not be corrupted souls. Yet here Tolkien found another stumbling block: according to his Catholic-influenced vision, Evil cannot create, only corrupt and destroy. So Morgoth couldn't have created Orcs, he must have used existing souls as raw material.

Tolkien never resolved this conundrum.

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[1] someone in another comment argued quite convincingly that Catholics at times had no trouble murdering other Christians over doctrinal affairs, so let's add a qualification here: "Tolkien's Catholic-influenced morality, which was his own nonetheless".