Why did Manwë not understand the concept of Evil?

Question

Manwë and Morgoth were brothers from their very creation. Morgoth whom created evil and wrote into the Music of the Ainur, chords of discord that would affect the events of Arda to come, had corrupted many Spirits into his service and would lead them to war against the Valar in the early years and later against the Children of Ilúvatar in the lands of Beleriand in the First Age.

Manwë on the other hand was blissful and the wisest of the Ainur who came to dwell within Eä. He had no understanding of evil and therefore after the ages that Morgoth was imprisoned showed mercy to his brother even though Morgoth's malice was stronger than ever. How could Manwë not have foreseen the deception that his brother would soon play upon him or the Noldor Elves for that matter? How could Manwë have fallen for Morgoth's deceit in his hearing for his release when it should have been clear he was playing him and the other Valar for fools save Oromë, Tulkas and Ulmo?

Why did Manwë not understand the concept of evil and wasn't he aware of such deception within his realm of Valinor?

Answer 1

I think you have put your finger on a paradox, but I think you underestimate Manwë, though you have good reason to, since Tolkien says in The Silmarillion:

For Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Ilúvatar, Melkor had been even as he; and he saw not to the depths of Melkor's heart, and did not perceive that all love had departed from him for ever.

But I think this may overstate the case if you read "comprehend" in its most common sense of "understand". "Comprehend" also has a second major sense of "encompass" or "embrace" or "take in" and I think this is the sense Tolkien uses here. Manwë cannot internalize evil. (This interpretation is supported by the fact that the whole Silmarillion is written in somewhat archaic language, and this sense of "comprehend" is the older, also, and so in keeping with the language used.)

To suggest that good can not understand evil (but evil can understand good) is simply wrong: For example, Gandalf's whole strategy of sending the Ring to Mount Doom is based on the opposite. He understands Sauron, but Sauron does not understand him:

Well, let folly be our cloak, a veil before the eyes of the Enemy! For he is very wise, and weighs all things to a nicety in the scales of his malice. But the only measure that he knows is desire, desire for power; and so he judges all hearts.

And later Galadriel says:

I say to you, Frodo, that even as I speak to you, I perceive the Dark Lord and know his mind, or all of his mind that concerns the Elves. And he gropes ever to see me and my thought. But still the door is closed!

Lastly, there's a second major point in Manwë's defense. Melkor was the one created being who was created Manwë's superior. If Manwë — or anyone, including ourselves — is going to misjudge anyone it's going to be people who are smarter than us, more persuasive than us, possessing superior gifts to us. (And of this Manwë had no doubts.)

I don't find it surprising that Manwë was fooled by Melkor.

Answer 2

Manwë had promised to release Melkor, and to break his promise would corrupt Manwë and turn him into a second Melkor.

The weakest and most imprudent of all the actions of Manwë, as it seems to many, was the release of Melkor from captivity. From this came the greatest loss and harm: the death of the Trees, and the exile and the anguish of the Noldor. Yet through this suffering there came also, as maybe in no other way could it have come, the victory of the Elder Days: the downfall of Angband and the last overthrow of Melkor.

Who then can say with assurance that if Melkor had been held in bond less evil would have followed? Even in his diminishment the power of Melkor is beyond our calculation. Yet some ruinous outburst of his despair is not the worst that might have befallen. The release was according to the promise of Manwë. If Manwë had broken this promise for his own purposes, even though still intending “good”, he would have taken a step upon the paths of Melkor. That is a perilous step. In that hour and act he would have ceased to be the vice-regent of the One, becoming but a king who takes advantage over a rival whom he has conquered by force. Would we then have the sorrows that indeed befell; or would we have the Elder King lose his honour, and so pass, maybe, to a world rent between two proud lords striving for the throne? Of this we may be sure, we children of small strength: any one of the Valar might have taken the paths of Melkor and become like him: one was enough.

_{The Nature of Middle-earth - "Ósanwe-kenta"}