The fact they were brought back means they were not dead.
That's not really true. Trek has used science to bring people back from the dead several times. That doesn't really mean the moral or legal definition of "dead" is wrong.
If Seperating Tuvix is murder then refusing to seperate Tuvix can be nothing less than choosing to kill them. If they were alive inside Tuvix, then Tuvix by extension can be alive inside them.
The "deaths" of Tuvok/Neelix/Tuvix are somewhat philosophical questions. Maybe they're dead, maybe they aren't.
But if Tuvok and Neelix are not dead, then the only person in any position to speak for their wishes is Tuvix. And Tuvix said that, through the fusion, Tuvok and Neelix had reached a new state of happiness that was previously unknown and unknowable to them.
If Tuvok and Neelix are dead, then they have lost the right to be prioritized over the living.
Tuvix identified himself as his own unique being. A third identity. If Tuvok and Neelix could be restored at no cost, then that would be one thing, but their restoration comes at what Tuvix (and Voyager's own Chief Medical Officer) perceives as the death of a living person who explicitly does not consent to be sacrificed.
If Tuvok and Neelix are dead, then the point of death was the accident. It's nobody's fault. If Tuvok and Neelix can be saved through Tuvix's death, then that's blood on Janeway's hands (should she choose to do so, which she did), and that's murder. There was no "the real murder would have been to
not shoot Tuvix in the back of the head".
Like, if two people are mortally wounded in a traffic accident, because their car hit a pole after they swerved to avoid a negligent jaywalker, then that jaywalker owes his life to the two people in the car. But the authorities cannot order the execution and organ harvesting of the jaywalker to save the lives of the people in the car (and it's especially bad when the #1 authority does it over the objections of the only two people qualified to make the ethical decisions, because the authority holds the power and just so happens to be friends with the two victims and doesn't give a shit about the jaywalker who she just met).
Or, if you were to assert that the you of today is an entirely different person from the you of one year ago, that does not mean that the you of one year ago is "dying", and that the only way to "stop murdering the you of one year ago" is to club you on the head and give you amnesia. If the you of today is the you that exists, then you're the one who gets the right to your being. The you of one year ago still exists inside of you, and is not in any real danger, while giving you amnesia would murder/destroy the you of today. And no, clubbing you on the head and destroying the current you
would not allow the current you to live on inside your past self, as your past self lives on inside you.
Janeway murdered/destroyed Tuvix because she didn't like what he had become.