So this is Star Trek. Being "not dead in a classical sense" applies to everyone who ever steps foot on a transporter. It's all sci-fi nonsense, so I really don't think we can apply any aspect of it to questions of ethics. And we don't have to, actually.

"Newborn" was not the right word. Tuvix was a new person that "began life" at the moment of the transporter accident. Yes, he was the combination of two separate people, but as he demonstrates throughout the episode, he is his own person, and in many ways, better at vital tasks than either of his "ancestors." He was a unique and valuable sentient life-form deserving of life.

I maintain there was no ethical dilemma. The question was never: "do you LET two people die to SAVE one." Again: Tuvix was never in any danger to be "saved" from. If no one does anything, then Tuvak and Neelix would have died, and Tuvix will continue living. If someone does something, then Tuvix dies. Full stop. The only enticing part about it is that those two dead people now become alive again through sci-fi wizardry.

The ethical dilemma would have been more high-stakes if, say, Tuvix would have died in a month anyway due to [insert technobabble here]. Then someone doing something might be more of an ethical dilemma. But as it stands, the only ACTION Janeway could have taken would be to end a life. The INACTION would be to allow Tuvix to continue living.

The classical ethics thought-experiment, The Trolley Problem, is fundamentally a question of agency and acts of will. The trolley is going to kill 5 people. Pull a lever and it diverts the trolley to the other track where it kills 1 person. Do you pull the lever and kill 1 to save 5? Does not pulling the lever make you responsible since you are there? And this is more relatable to this conversation: what if instead of pulling a lever, you have to push someone onto the tracks to save the 5?

But in this situation, the trolley has already killed 2 people through no action of your own (or anyone else's; it was an accident). Tuvix is alive and well and is in no danger of they trolley. But through sci-fi nonsense, if you pick up Tuvix and throw him onto the tracks, the trolley will kill him and somehow bring Tuvak and Neelix back to life. The ethics are clear.

Full disclosure, I consider Janeway to be one of my favourite captains of all time, if not my very favourite. I LOVE Janeway. I think she was fantastic and I'm devastated that Prodigy was cancelled. However, I think in this particular situation, she acted unethically. I don't blame her, I probably would have done the same thing for my beloved friends. But she did not follow classical nor Starfleet ethics in this case. Her action caused the death of a sentient life (new and unique in all the universe!) for the sake of two dead (whatever that means) friends. IMHO she made a mistake, but she's human and the mistake just makes her more relatable for it.