From The Silmarillion/Of the coming of the elves
Thus it was that when Nahar neighed and Orome indeed came among them[Quendi], some of the Quendi hid themselves, and some fled and were lost. But those that had courage, and stayed, perceived swiftly that the Great Rider was no shape out of darkness; for the light of Aman was in his face, and all the noblest of the Elves were drawn towards it.
But of those unhappy ones who were ensnared by Melkor little is known of a certainty. For who of the living has descended into the pits of Utumno, or has explored the darkness of the counsels of Melkor? Yet this is held true by the wise of Eressea, that all those of the Quendi who came into the hands of Melkor, ere Utumo was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves, of whom they were afterwards the bitterest foes. For the Orcs had life and multipled after the manner of the Children of Iluvatar; and naught that had life of its own, nor the semblance of life, could ever Melkor make since his rebellion in the Ainulindale before the Beginning: so say the wise.
So, at least when Tolkien wrote The Silmarillion, he meant for the Orcs to have been corrupted ( in the physical and psychological sense) from unhappy elves, not necessarily evil elves.
Christopher Tolkien notes in Unfinished Tales 'but this was only one of diverse speculations on the origin of the Orcs.
So, it would seem, JRRT couldn't quite make up his mind on this one.