You've missed the point. A commander can order a fellow officer to do their duty to the crew even if the consequence is death; so Troi can order the chief engineer to repair the warp drive even if it means fatal radiation poisoning. We cannot order the chief engineer to simply be killed.
Don't be absurd. Morality is more important than anything else; the entire point of life is to be moral. And besides, Tuvix was as competent an officer as Tuvok; killing him and replacing him with a copy of Tuvok does not actually benefit ship's efficiency.
No, you missed the point. It was pointed out to Deanna by Riker , very very clearly, that her first duty IS TO THE SHIP. The ship. Deanna's whole trouble with the test was that she was having difficulty sending a person, any person, to their death. Morals have nothing to do sometimes with command decisions when the first duty is to the ship.
To quote Deanna, right after the test, "That's what this is all about wasn't it? To see if I would order someone to their death?"
Riker: "That's right."
Riker said nothing about being prepared for only command decisions regarding Engineering deaths, but to be prepared to make what he called, the "hard choice."
Getting back to Janeway and Tuvix. Janeway had to make a hard choice, and one way or another the "moral" high ground simply wasn't available to her. Morally, like she herself said and Kes agreed with, the captain had a moral obligation to Tuvok's family and Neelix's. She had a command obligation to the ship.
And why is Tuvix morally innocent. It is the moral obligation of any Starfleet officer to sacrifice their lives for another. Tuvix had an opportunity to save the lives of two men. I counter that it was Tuvix who was as morally wrong in this argument on whether he should be separated or not.
Here's another question I'm curious about. Why is Tuvix and Janeway's decision to seperate him any different than Captain Archer's order to use Sim, Trip Tucker's clone, to sacrifice his life in order to save his Chief Engineer? Why do we not have a Archer's Decision To Kill Sim thread? It's the same situation.
And morally, is not the prime directive also an immoral directive? The point being that not all command decisions can have a clear cut moral high ground to live up to?
You must have a...resenment towards Captain Janeway to brand her a murderer when every Captain from Kirk to Archer has made those kind of choices themselves.