Evil is not a zero sum resource in Tolkien's world that must be apportioned between individual beings. This is made evident throughout his work, for example by descriptions of really evil beings like Melkor/Morgoth or Sauron corrupting others. In fact, Sauron, once an angelic fire spirit (a Maia) in the service to Aulë back when the world was being created in Tolkien's legendarium (see "The Valaquenta" in The Silmarillion), was corrupted by Melkor.
Another example is the creation of the race of Orcs (as published in The Silmarillion, a subject Tolkien was torn about in later Letters):
So it came to pass, some years ere the coming of Oromë, that if any of the Elves strayed far abroad, alone or few together, they would often vanish, and never return… But of those unhappy ones who were ensnared by Melkor little is known of a certainty… Yet this is held true by the wise of Eressëa, that all those of the Quendi [i.e. Elves] who came into the hands of Melkor, ere Utumno 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 multiplied after the manner of the Children of Ilúvatar; and naught that had life of its own, nor the semblance of life, could ever Melkor make since his rebellion in the Ainulindalë before the Beginning: so say the wise. And deep in their dark hearts the Orcs loathed the Master whom they served in fear, the maker only of their misery. This it may be was the vilest deed of Melkor, and the most hateful to Ilúvatar. (emphasis added)
—The Coming of the Elves and the Captivity of Melkor, The Silmarillion
Melkor was not made less evil, or more good by creating the Orcs, but their creation was a manifestation of his evil. Indeed, Tolkien described all evil in the world as having its origin ultimately in Melkor. Tolkien never describes such acts as redistribution of evil as a finite resource, but as the creation of new evil where it did not exist before.