It isn't a matter of weighing one life against two others. The two others are already dead. Ethics doesn't work with time reversal. Part of the right to life is because that life already exists, as opposed to will exist, or has existed. It's a decision very much in the present. Creating lives and destroying lives are not ethically balanced opposites.
Well, ethics can certainly be applied to history, and the lessons that people like Janeway draw from it, both their personal history and their larger history (i.e. that of Earth, of the Federation, and so forth). That humans on Earth at one point waged wars of conquest and hatred would seem just as ethically wrong to Janeway as it does to us in the real world.
That life exists shouldn't be the sole criteria for determining whether it's worth saving or not. If it were, then nobody who was suffering in horrible pain would have the right to ask for death. Quinn would have had no right to request that his immortality come to an end, even though it had become nothing but a burden to him.
In this situation, unethical actions bring death. Ethics doesn't obligate action. Inaction (leaving things as they are) is generally ethical. Leaving Tuvok and Neelix dead and Tuvix alive is ethical.
Forcefully turning back the clock to how things were before ignores the fact that it brings death, which is unethical. Leaving Tuvok and Neelix dead, and Tuvix alive doesn't bring death.
- The accident killed Tuvok and Neelix. Their lives are written off as misfortune.
- Tuvix was created. Tuvix had a right to life. He wasn't responsible for the death of Tuvok and Neelix and had no ethical obligation to them, or anybody else.
On the contrary, the fact that Tuvix's "birth" was accidental does not make it
ethical, since it required Tuvok and Neelix to be "killed." That Tuvix was a unique entity is not in question, but as a combination of those two individuals, I very much dispute the claim that he had no ethical obligation to them or to their relationships with the rest of the crew.
If we're to accept the logic that killing Tuvix is unethical to revive two "dead" crewmembers, then logically his birth must be just as unethical since it couldn't have happened without those deaths. It assumes Tuvok and Neelix had no right to life. I think Tuvix's desire to survive was partly motivated out of fear, because he didn't want to die. And he may not have been willing to accept the consequences that his existence was having on Kes, Janeway and others. It's an understandable reaction, but it's also a selfish one.