Say we were to get any random Demon of the burning legion, and forcibly cleanse them of fel and make them be normal mortals again. There would be no problem, but when Xe’ra tried to do the same thing to Illidan, its seen as some evil thing.
Context is important, Illidan was in fact, a demon, who’s methods of doing things were only slightly better than that of the Burning Legion it self. The stuff he did in Outlands was completely indefensible in terms of morality, what good does it do if you’re fighting the legion if you’re out enslaving people, corrupting them, terrorizing them, and mass killing them.
Xe’ra’s choice to purge the demon out of Illidan was the morally correct one.
To me, defending Illidan’s “Rights” sounds as absurd as if some one were to say during the movie the Exorcist “That priest is bad, demons have rights!”