That wasn't up to Janeway, or at least it shouldn't have been. What Janeway did in this episode as far as I'm concerned is just as bad as if she had killed a random crew member in Phage to give Neelix new lungs.So Neelix and Tuvok don't deserve to live?By murdering an innocent man.
I'd say there's a difference. Tuvix wasn't some random crewmember; he only existed because of the loss of Tuvok and Neelix, because he's using their bodies/molecules/lives to exist. So I can see the argument that once Tuvok and Neelix can be restored, they are entitled to get their bodies/molecules/lives back, and Tuvix is not entitled to keep the bodies/molecules/lives of those two innocent people, even to preserve his own life.