I'm not hear to debate anyone, but if you think it is ok to kill a currently living being to resurrect a dead being, then you are fucked in the head. Tuvok and Neelix died painlessly and unaware in an accident. Tuvix was murdered, and was made fully aware of their fate beforehand, to the point where they even begged to be spared.
I think from their perspective Tuvok and Neelix weren't "dead", which was why they were more inclined to "correct" the situation at hand and save their crewmates while they still had the chance to do so.
Regardless, it's a fucked up decision, I don't envy it.
The episode did its job challenging viewers with the question, because people still argue about this today. But to me there's an actual, unambiguous answer: 4.823 seconds after transport autosequence initiation, when the emitter array completed the materialization cycle.
Moreover, Neelix and especially Tuvok as an enlisted Starfleet officer consented to the risk of death that comes with serving aboard a starship. And that's exactly what happened to them. Tuvix was never given the choice and was murdered for having had the bad fortune to be born aboard Voyager. So much for seeking out new life.
It's not an equation to be worked out. It simply boils down to respecting the wishes of a currently living and conscious being. Otherwise anyone's life could be forfeit based purely on some arbitrary valuation of what that life is worth. Why don't we just harvest your organs and give them to people we deem more useful, ya know?
If I had come about through the unwilling merger of two people, and my death could restore those people, it's probably ethical to kill me to make it happen.
I don't think it's necessarily reasonable to call the two component people dead either. Death is a not a particularly well defined term, but we don't tend to apply it to people who might get better.
Why don't we just harvest your organs and give them to people we deem more useful, ya know?
The knowledge that you live in a society where you could be legally killed at any point for the greater good, and the resultant fear and uncertainty probably would cause more harm overall than doing so could actually alleviate.