Tolkien apparently was wrestling with the notion late in life -- the notion of orcs formed from the ground, animated by Morgoth's will, severed into little self-willed chunks. One reason Morgoth lost after the Valar came was that this weakened him.
To be sure, the concept of splitting up the soul into independent beings that can fight each other is not easily made metaphysically sound. But if there were merely evil men, they could at an angry -- proud -- greedy moment, accept a magic offered to them to ensure that they remain strong in will and do not give up their desire for any passage of time or any pleading, even by their nearest and dearest. Or even if they hung out too long in the wrong sort of place, they could grow fixed in their will. Or even transform. A miser turning slowly into a dragon to keep watch over his gold. . . .
If you wanted them to be born evil, a sorcerer, even in a world without reincarnation, could offer the deal to vengeful, or otherwise evil ghosts, to be born in his artificially created race, with sorcery to keep them concentrated on their aim without letting the flesh cause them to waver.
Or, of course, a race of sociopaths. May inspire philosophical observations in the characters about whether they are really evil, but they are certainly devoid of conscience.