[William Cloud Hicklin]

Tolkien was at great pains to avoid a Manichaean world. It's inherent in his work that Power Corrupts- it's not really possible in Fallen Arda for there to be a powerful wholly Good being (except Manwe, who, tellingly, is very largely passive).

JRRT as above defined Absolute Evil as Zero- he indicated in one essay that Morgoth had reached a stage of 'nihilistic madness' and, left to his own devices, would have gone on raging until all Ea was reduced to chaos, and continued raging because the Chaos was not of his own making.

Evil in Tolkien is, I think ultimately Selfishness: greed, imposition of will, absence of compassion, ego, solipsism.

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The absence of religious observance in LR (not Silmarillion) is really very easily explained: Tolkien couldn't conceive his Good peoples as being pagan or anything inconsistent with Judaeo-Christianity. Therefore he felt it was safer to make them largely cult-less 'pure monotheists.' Temples and priests and the like were for the minions of Sauron, who ruled as a God-King.