'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. Who among the Eldar hold that the captivity of Melkor in Mandos (which was achieved by force) was either unwise or unlawful? Yet the resolve to assault Melkor, not merely to withstand him, to meet violence with wrath to the peril of Arda, was taken by Manwe only with reluctance. And consider: what good in this case did even the lawful use of force accomplish? It removed him for a while and relieved
Middle-earth from the pressure of his malice, but it did not uproot his evil, for it could not do so. Unless, maybe, Melkor had indeed repented. But he did not repent, and in humiliation he became more obdurate: more subtle in his deceits, more cunning in his lies, crueller and more dastardly in his revenge. The weakest and most imprudent of all the actions of Manwe, 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 Manwe. If Manwe 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-gerent 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.'
JRRT
Just because you can't comprehend 
