According to the episode, Tuvix was born through a process called symbiogenesis. He wasn't created, he didn't kill 2 people to create himself, he was *born*. Symbiogenesis is a form of procreation.
This consideration seems to be a much firmer basis on which to make a reasoned determination of the matter rather than "that's a tough one", "it could go either way", or going along with those that have long since punched their ticket on the Janeway Psycho Express.
Bearing in mind that this comes from someone who is a scientific idiot savant, minus the savant, I would say look at what this canonical explanation implies. Symbiogenesis is an evolutionary process that points toward cooperation in the synthesis of two different organisms in creating a new and unique one. This, I gather, supplements, but not necessarily supplants, the Darwinian paradigm of the primacy of competition as a driver. Now while in this situation a distinct life has been created, I am not sure I would define it as procreation. Certainly the act that brought Tuvix about was not an elective one that Neelix and Tuvok discerned, cogitated on, and then made an affirmative decision to execute. The examples of symbiogenesis that I have gleaned in my cursory look at the subject do not extend to higher forms and definitely not sentient beings.
Yet, it seems that there can be a wide variety of causative factors that lead to this outcome ranging over a spectrum of time frames, environmental stresses, etc. It would seem that the chance occurrence that befell N & T would not be out of character to be considered a valid expression of the process and therefore negate the argument of those who might claim Tuvix as an inconsequential "accident" with no legitimate standing to be seen as a co-equal to any one else in Voyager's complement.
While his integrity and inherent worth as an individual cannot then be diminished, I think it is possible to convey more rights than are warranted to Tuvix that would confer the status of a truly self-determining being, as contradictory at that would seem. He cannot nor, I believe, does he ever claim to unambiguously speak for either of his progenitors, each of whose individual hopes, desires, and aspirations would inevitably become increasingly diffused by Tuvix's burgeoning consciousness.
While only a hope initially, the Doctor seeks to find a means to return Neelix and Tuvok to their living forms and not just as a symbolic existence. I suppose I would ask that regardless what stratagem he might have ultimately devised to attain this goal, would that process carry any less of a natural rationalization than the one that was randomly ordered? Ultimately, I am not of the opinon that Neelix and Tuvok no longer existed, but simply were not currently present
. As such, the effort to bring them back trumps Tuvix's right to unilaterally author his fate. At the same time, I would urge that whatever could be accomplished to maintain him as an autonomous entity be efforted as strenuously and for as long as possible. If for some reason cloning was not an option, have him put into stasis until such time as a solution was available otherwise and be done with it.