(* [footnote to the text]
One of the reasons for [Morgoth's] self-weakening is that he has given to his 'creatures', Orcs, Balrogs, etc. power of recuperation and multiplication. So that they will gather again without further specific orders.
Part of his native creative power has gone out into making an independent evil growth out of his control.)
(...)
Out of the discords of the Music - sc. not directly out of either of the themes,(17) Eru's or Melkor's, but of their dissonance with regard one to another - evil things appeared in Arda, which did not descend from any direct plan or vision of Melkor: they were not 'his children'; and therefore, since all evil hates, hated him too. The progeniture of things was corrupted. Hence Orcs? Part of the Elf-Man idea gone wrong.
(...)
See 'Melkor'. It will there be seen that the wills of Orcs and Balrogs etc. are part of Melkor's power 'dispersed'.
Their spirit is one of hate. But hate is non-cooperative (except under direct fear). Hence the rebellions, mutinies, etc. when Morgoth seems far off. Orcs are beasts and Balrogs corrupted Maiar. Also (n.b.)
Morgoth not Sauron is the source of Orc-wills. Sauron is just another (if greater) agent. Orcs can rebel against him without losing their own irremediable allegiance to evil (Morgoth).
(...)
The origin of the Orcs is a matter of debate. Some have called them the Melkorohini, the Children of Melkor; but the wiser say: nay, the slaves of Melkor, but not his children; for Melkor had no children.(1)
Nonetheless, it was by the malice of Melkor that the Orcs arose, and plainly they were meant by him to be a mockery of the Children of Eru, being bred to be wholly subservient to his will and filled with unappeasable hatred of Elves and Men.
(...)
Melkor was impotent to produce any living thing, but skilled in the corruption of things that did not proceed from himself, if he could dominate them. But if he had indeed attempted to make creatures of his own in imitation or mockery of the Incarnates, he would, like Aule, only have succeeded in producing puppets: his creatures would have acted only while the attention of his will was upon them, and they would have shown no reluctance to execute any command of his, even if it were to destroy themselves.
But the Orcs were not of this kind.
They were certainly dominated by their Master, but his dominion was by fear, and they were aware of this fear and hated him. They were indeed so corrupted that they were pitiless, and there was no cruelty or wickedness that they would not commit; but this was the corruption of independent wills, and they took pleasure in their deeds. They were capable of acting on their own, doing evil deeds unbidden for their own sport; or if Morgoth and his agents were far away, they might neglect his commands. They sometimes fought [> They hated one another and often fought] among themselves, to the detriment of Morgoth's plans.
Moreover, the Orcs continued to live and breed and to carry on their business of ravaging and plundering after Morgoth was overthrown.
...
They might have become irredeemable (at least by Elves and Men), but they remained within the Law. That is, that though of necessity, being the fingers of the hand of Morgoth, they must be fought with the utmost severity, they must not be dealt with in their own terms of cruelty and treachery. Captives must not be tormented, not even to discover information for the defence of the homes of Elves and Men. If any Orcs surrendered and asked for mercy, they must be granted it, even at a cost. This was the teaching of the Wise, though in the horror of the War it was not always heeded.
It is true, of course, that Morgoth held the Orcs in dire thraldom; for in their corruption they had lost almost all possibility of resisting the domination of his will. So great indeed did its pressure upon them become ere Angband fell that, if he turned his thought towards them, they were conscious of his 'eye' wherever they might be; and when Morgoth was at last removed from Arda the Orcs that survived in the West were scattered, leaderless and almost witless, and were for a long time without control or purpose.
This servitude to a central will that reduced the Orcs almost to an ant-like life was seen even more plainly in the Second and Third Ages under the tyranny of Sauron, Morgoth's chief lieutenant. Sauron indeed achieved even greater control over his Orcs than Morgoth had done. He was, of course, operating on a smaller scale, and he had no enemies so great and so fell as were the Noldor in their might in the Elder Days.
But he had also inherited from those days difficulties, such as the diversity of the Orcs in breed and language, and the feuds among them; while in many places in Middle-earth, after the fall of Thangorodrim and during the concealment of Sauron, the Orcs recovering from their helplessness had set up petty realms of their own and had become accustomed to independence. Nonetheless Sauron in time managed to unite them all in unreasoning hatred of the Elves and of Men who associated with them; while the Orcs of his own trained armies were so completely under his will that they would sacrifice themselves without hesitation at his command.*
And he proved even more skilful than his Master also in the corruption of Men who were beyond the reach of the Wise, and in reducing them to a vassalage, in which they would march with the Orcs, and vie with them in cruelty and destruction.
(* [footnote to the text] But there remained one flaw in his control, inevitable.
In the kingdom of hate and fear, the strongest thing is hate. All his Orcs hated one another, and must be kept ever at war with some 'enemy' to prevent them from slaying one another.)
...
And
though Angband has fallen and Morgoth is removed, still they come forth from the lightless places in the darkness of their hearts, and the earth is withered under their pitiless feet.