How much of a difference is there between being naturally bad and irredeemably bad? (Tolkien)

"[Orcs] would be Morgoth’s greatest Sins, abuses of his highest privilege, and would be creatures begotten of Sin, and naturally bad. (I nearly wrote ‘irredeemably bad’; but that would be going too far. Because by accepting or tolerating their making—necessary to their actual existence—even Orcs would become part of the World, which is God’s and ultimately good.)"

Reading this quote makes me ponder. How much does it actually change?
Is killing a naturally bad orc more evil than killing irredeemably evil orc?
Does the theoretical possibility of redemption make naturally bad person more evil, compared to somebody whose evil nature is unchangeable.