I want to live!
- EvilKirk’s last words
Whatever else you want to say about EvilKirk, it’s pretty clear that he didn’t want to be merged back into the single Kirk. Despite this, there is no shortage of reasons why it was a good idea to merge the Kirks: the Enterprise needed its CO back, GoodKirk wanted to do it, and it seems possible that the strain of remaining split would have eventually killed EvilKirk anyways. However, the fact remains that EvilKirk did not consent to the procedure which ended his existence.
Clearly the circumstances here are quite different and there’s basically no argument to be made that allowing EvilKirk to continue to exist would benefit any involved party, EvilKirk included. But for the purposes of this comparison, the only fact that really matters is that EvilKirk was just as passionate about his desire to continue existing as Tuvix was.
Yet—and it’s obvious where I’m going with this—“Spock murdered EvilKirk” is not a meme.
So what gives? Did Spock murder EvilKirk or not? If yes, why does he get a pass while Janeway is condemned?
That is true and to @ValueSubtracted’s point, speaks to the fundamental difference between the morality tale that each episode is setting up for our consideration. “The Enemy Within” aims to make the viewer uncomfortable by suggesting that Kirk’s decisiveness is derived from his “evil” half and isn’t offering any commentary on the personhood of the “evil” half. So much so that they short circuit that possibility by slapping an expiration date on EvilKirk.
Best I can give you is that it’s still technically murder if you kill a condemned man.
Killing two people who are both destined to die in the very near future to save one who will live for a considerably longer period and save a greater number of lives would be the right thing to do from a utilitarian standpoint. Tuvix, meanwhile, is a healthy being who was competent to discharge his duties and posed no threat.
Consequently, while there is an argument to be made that killing one person to revive two others is a net benefit, the burden of suffering on that one person is extreme, and whether or not it is outweighed by the positive nature of the two others returning to live is very much a matter of individual outlook.
It is also worth noting that Tuvok and Neelix as they existed before could be considered “already dead” as a result of their combination into a single entity. Thus you could argue that what actually happened is that Tuvix “died” so that clones of the deceased Tuvok and Neelix could be created from him. Admittedly this is a shaky argument given the whole “do transporters actually kill people in-universe” thing.