Imagine that Tuvix is the orchid speaking while holding Tuvok and Neelix hostage

Of course, it doesn't literally have to be the orchid -- although it's thought-provoking to consider that angle. (An orchid suddenly gets super-evolved with the ability to speak and walk -- damn straight it's gonna want to live and would happily lie to do it.) But the point to consider is the source of various claims that are made about the situation.

Critically, Tuvix claims that he speaks for both Neelix and Tuvok -- that both of them are present and both of them are happy being merged.

But that's actually a somewhat bold claim, especially the second point. Given what we know of Tuvok and Neelix, it seems hard to imagine either of them voluntarily signing up for this -- certainly given their mild personal animosity, but also because we really have no evidence to suggest that either of them feel "incomplete". Like, Tuvok really doesn't seem that interested in becoming more in touch with his emotions; Neelix is a little more debatable, but even he doesn't really seem like he is longing to be more logical or serve in Starfleet.

So Tuvix is making at least one implausible claim; that claim is important because, if it were accurate, then yes, Janeway's decision goes against the wishes of Tuvok, Neelix, and Tuvix, which would be bad.

For the sake of argument, imagine then that Tuvix is actually neither Tuvok nor Neelix, but is the orchid, with access to Tuvok and Neelix's memories, knowledge, and personality -- and maybe even holding mental versions of their persons as silent captives. The orchid would have incentive to lie, and would have all the means at its disposal to engage in a convincing deception.

In that scenario (and certainly if Janeway discovered this to be true), it seems pretty straightforward that deintegrating Tuvix is the right thing to do: Tuvok and Neelix are being held captive with no ability to advocate for themselves. The orchid, while surely benefiting from the situation, does not have the right to usurp the autonomy of Tuvok and Neelix.

It is, of course, unknowable whether the orchid was in fact holding Neelix and Tuvok hostage. But even if we set aside the orchid, and take the more conventional interpretation that Tuvix is a composite individual arising from the transporter combination of Tuvok and Neelix, I think the above argument still holds.

Tuvix is not Tuvok nor Neelix -- that much is clear externally as well as by his own account. Tuvix (who is not Tuvok nor Neelix) claims to speak for both Tuvok and Neelix and makes an assertion that would seem out of character for both individuals. Tuvix makes claims that blatantly serve his self-interest. Tuvix advocates action that benefits him, and which has an unknowable impact on Tuvok and Neelix but either way denies them their autonomous existence.

Whether "Tuvix" is a malicious masquerading hyper-evolved orchid, or a genuine composite individual speaking from his unique perspective, it doesn't really matter. There is no usable evidence of consent from either Tuvok or Neelix, and there are ample reasons to believe that they would not consent to this situation. Janeway has no way of knowing that Tuvok and Neelix aren't both screaming inside of Tuvix, demanding to be freed -- a scenario that does seem likelier true than not.

Tuvix's death is a tragedy, as was his birth.