The trivia items below may give away important plot points.
In the original production order, Skin of Evil preceded Symbiosis. The order was changed because Denise Crosby had asked Gene Roddenberry if she could remain on the show for Star Trek: The Next Generation: Symbiosis (1988). She had gotten a hold of the script, and thought it was interesting, so she asked if her character's death could be delayed for one more episode. Roddenberry granted her request, and had the airing orders for the two episodes switched, so there would be no continuity issues. This explains why she's waving goodbye at the end of Symbiosis, her last filmed episode.
There was some debate among the writers over the final fate of Armus. Gene Roddenberry ultimately mandated that Armus not be killed, believing that as human beings, no final moral judgment be made on any creature that was encountered.