I think the "Man Trap" Was a brilliant episode. I don't care what anyone thinks the way to handle the Last Salt Vampire was or was not ethical. In some situations it comes down to the laws of Nature.The fact that morally questionable things are presented in any Star Trek does not bother me. Star Trek has had genocide, and murder, and rape and all other kinds of moral issues that are not presented in the best way, or even in a way that invites comment. First thought that sprung to mind was "The Man Trap," with the only solution to kill a creature that is the last of its kind. Does that invoke such moral outrage and condemnation?
For me, I don't expect Star Trek to get it morally right or present the issue in a perfectly acceptable manner. What matters is how the audience engages with the material.
If the last of a race of beings is killing your crew, and there is only the choice of survival, Me vs you. It's the food chain, or unbridled nature that is being displayed. Kill or be killed. The Moral question of Genocide is usurped by this fundamental law of nature... "Survival" which is what the episode ultimately was preaching. That in the end, no matter how much trying to negotiate or convince the last salt vampire to stop trying to kill the crew, it could not go against her nature, and ultimately that nature is what ended with her being killed. Tragic, but if not for the killing and the lack of salt, the species would have kept going until she died of natural causes. So the moral of the story is that the species in a Darwinian sort of way, dies out due to it's nature, and that is just how some things in life are. Some species are destined to eventually die out. Usually they are predators that once the food sources are exhausted, and no alternatives are available, die off. IN the case of the Salt Vampire, she found a new source, but it is a high stakes play, because she the salt vampire left no choice to the crew, kill or be killed. The struggle of the salt vampire to curb it's own nature was also a feature of the program. I never understood why people (not you in particular) try to break down episodes to a simple it's this or that, without taking into account the full context of it, and subtlety of the message and acting ques within.