Det var inte upp till "Valaren", det var enbart Manwe beslut.

Before the gates of Valmar Melkor abased himself at the feet of Manwe and sued for pardon, vowing that if he might be made only the least of the free people of Valinor he would aid the Valar in all their works, and most of all in the healing of the many hurts that he had done to the world. And Nienna aided his prayer; but Mandos was silent.

Then Manwe granted him pardon; but the Valar would not yet suffer him to depart beyond their sight and vigelence, and he was constrained to dwell within the gates of Valmar.

But Ulmo was not deceived, and Tulkas clenched his hands whenever he saw Melkor his foe go by; for if Tulkas is slow to wrath he is also slow to forget. But they obeyed the judgement of Manwe; for those who will defend authority against rebellion must not themselves rebel.

Silmarillion - Of Feanor och Unchaining of Melkor.

När det gäller Manwe, går det tillbaka till din tidigare fråga om varför han inte förstod det onda:

Varför förstod Manwe inte begreppet ondska

Det var Manwes beslut via Eru * s vilja

Manwë förutspådde att ut ur detta onda "nytt gott skulle komma".

Det är ganska tydligt från den publicerade Silmarillion att Manwë var den avgörande omröstningen när det gällde Melkors re-trial. Men Manwë hade mer kunskap då vi kan uppfatta, och även om hans beslut kan se fel, var det rätt beslutet att tillåta god tillväxt i världen, det var också i linje med Will of Eru Iluvatar, skaparen

Bakgrund

När Melkor var kedjad. Han dömdes till "tre åldrar" (Sil, III) av fängelse i Mandos Halls innan han skulle få en ny rättegång. Valaren var rena varelser tvingades att hålla sitt ord, och därför efter det att tiden ansågs förfluten, återkom Melkor återigen inför Manwë,

For it came to pass that Melkor, as the Valar had decreed, completed the term of his bondage, dwelling for three ages in the duress of Mandos, alone. At length, as Manwë had promised, he was brought again before the thrones of the Valar.

Then Manwë granted him pardon; but the Valar would not yet suffer him to depart beyond their sight and vigilance, and he was constrained to dwell within the gates of Valmar.

... therefore in a while he was given leave to go freely about the land, and it seemed to Manwë that the evil of Melkor was cured. For Manwë was free from evil and could not comprehend it, and he knew that in the beginning ... Melkor had been even as he...
The Silmarillion - Chapter VI, Of Fëanor and the Unchaining of Melkor

Från den publicerade Silmarillion får vi tanken att Manwë hade blivit lurad återigen av Melkor's knep och att han genom svik hade funnit sin frihet och återigen kunde utföra sina machinations.

But, if we dare to attempt to enter the mind of the Elder King, assigning motives and finding faults, there are things to remember before we deliver a judgement. Manwe was the spirit of greatest wisdom and prudence in Arda. He is represented as having had the greatest knowledge of the Music, as a whole, possessed by any one finite mind; and he alone of all persons or minds in that time is represented as having the power of direct recourse to and communication with Eru. He must have grasped with great clarity what even we may perceive dimly: that it was the essential mode of the process of 'history' in Arda that evil should constantly arise, and that out of it new good should constantly come. One especial aspect of this is the strange way in which the evils of the Marrer, or his inheritors, are turned into weapons against evil.

Som Tolkien säger i uppsatsen hade Manwë mer kunskap än oss som läsare och var medvetet, antingen genom hans visdom eller kommunikation med Eru, att frisläppandet av Melkor var en nödvändig del av historiens process Arda". Denna idé upprepas på Osanwe-kenta (vänster längst ner på grund av att den är oerhört lång). I Ósawne-kenta diskuterade Tolkien betraktarens uppfattning om dumbommen i Manwë i sina beslut och den lätthet som Melkor lurade andra. Men professorn föreslår igen att Manwë inte kunde ha tvingat Melkor att prata sanningen, eller att den andra Valaren (genom telepati) som det skulle kunna använda ondska, lurar ondska, som Valaren inte kunde göra. Manwë var öppen för Erus tankar och utövade sin vilja, detta uppfattas vara större än visdom. Manwes åtgärder säkerställde att Eras plan skulle utvecklas. I en liknande förgäves, det var bara genom Erus ord att Manwë äntligen halshuggade Melkor och skickade sin ande för att vandra tomrummet.

If we speak last of the "folly" of Manwe and the weakness and unwariness of the Valar, let us beware how we judge. In the histories, indeed, we may be amazed and grieved to read how (seemingly) Melkor deceived and cozened others, and how even Manwe appears at times almost a simpleton compared with him: as if a kind but unwise father were treating a wayward child who would assuredly in time perceive the error of his ways. Whereas we, looking on and knowing the outcome, see now that Melkor knew well the error of his ways, but was fixed in them by hate and pride beyond return. He could read the mind of Manwe, for the door was open; but his own mind was false and even if the door seemed open, there were doors of iron within closed for ever.
How otherwise would you have it? Should Manwe and the Valar meet secrecy with subterfuge, treachery with falsehood, lies with more lies? If Melkor would usurp their rights, should they deny his? Can hate overcome hate? Nay, Manwe was wiser; or being ever open to Eru he did His will, which is more than wisdom. He was ever open because he had nothing to conceal, no thought that it was harmful for any to know, if they could comprehend it. Indeed Melkor knew his will without questioning it; and he knew that Manwe was bound by the commands and injunctions of Eru, and would do this or abstain from that in accordance with them, always, even knowing that Melkor would break them as it suited his purpose. Thus the merciless will ever count on mercy, and the liars make use of truth; for if mercy and truth are withheld from the cruel and the lying, they have ceased to be honoured.
Manwe could not by duress attempt to compel Melkor to reveal his thought and purposes, or (if he used words) to speak the truth. If he spoke and said: this is true, he must be believed until proved false; if he said: this I will do, as you bid, he must be allowed the opportunity to fulfill his promise.
The force and restraint that were used upon Melkor by the united power of all the Valar, were not used to extort confession (which was needless); nor to compel him to reveal his thought (which was unlawful, even if not vain). He was made captive as a punishment for his evil deeds, under the authority of the King. So we may say; but it were better said that he was deprived for a term, fixed by promise, of his power to act, so that he might halt and consider himself, and have thus the only chance that mercy could contrive of repentance and amendment. For the healing of Arda indeed, but for his own healing also. Melkor had the right to exist, and the right to act and use his powers. Manwe had the authority to rule and to order the world, so far as he could, for the well-being of the Eruhíni; but if Melkor would repent and return to the allegiance of Eru, he must be given his freedom again. He could not be enslaved, or denied his part. The office of the Elder King was to retain all his subjects in the allegiance of Eru, or to bring them back to it, and in that allegiance to leave them free.
Therefore not until the last, and not then except by the express command of Eru and by His power, was Melkor thrown utterly down and deprived for ever of all power to do or to undo.

Det här är en intressant fråga, och medan jag kan handwave flera teorier - gjorde jag just det medan jag försökte rama ett svar på den här frågan - i slutändan var ingen av dem övertygande. Jag är tvungen att dra slutsatsen att Valaren verkar ha skruvat upp. De gjorde ett (mycket stort) misstag.

Jag kan inte se hur det kan vara att Manwe, et al , var hoodwinked eftersom de var bra och därmed naiva. Att låta Morgoth gå var så uppenbart att ett misstag som jag misstänker mest, när de kom till den tiden i boken, tänkte på sig själva Nej nej! Är du inte upptagen!? Hur som helst tror jag inte det bra = > naivt, och Tolkien gjorde inte heller. (Jag tror att det finns en fråga där det här diskuteras, men jag har problem med att hitta det. Uppdatering: Jag tackar @suchiuomizu i hans svar för att hitta den: Varför förstod Manwe inte begreppet Ond? )

Har Iluvatar berättat för Manwe att låta honom gå som en del av en oändlig plan? Kanske, men Tolkien säger inte någonstans eller ens antydning till det, så jag tror inte att det är ett användbart svar.

Var det fated av musiken? Kanske. Silmarillion säger

Therefore he willed that the hearts of Men should seek beyond the world and should find no rest therein; but they should have a virtue to shape their life, amid the powers and chances of the world, beyond the Music of the Ainur, which is as fate to all things else...

Detta skulle innebära att Valaren (och Maiar och Elverna) styrdes av Öde, medan män inte var. Men vad det än betyder betyder överallt i boken Valar och Elver och Män alla ganska lika med självviljade varelser. Så jag köper inte heller det.

Morgoth använde sin Jedi Mind Trick på Manwe et al ? Detta är faktiskt den enda förklaringen som jag kan acceptera. Morgoth var den mest kraftfulla av Ainur - kraftfullare än Manwe eller någon av de andra - och jag kan sortera-möjligen-kanske acceptera att han sa " Jag är inte den mörka herren du är letar efter , "wiggled fingrarna och släpptes. Men jag gillar inte heller den förklaringen, heller.

Jag är tvungen att dra slutsatsen att svaret (i universum) är att Valaren gjorde ett misstag: De är bara mänskliga ...