I agree that Tuvix was a more likable and "better" character than either Tuvok or Neelix !!Her Choice in Tuvix REALLY turned me off to Janeway
Tuvix was a mostly impossible choice. I don't see how anyone can fault her for either decision to be made. Kill Tuvix and let Tuvok and Neelix live, or kill Neelix and Tuvok and let Tuvix live. Either way she's going to have to decide to kill somebody so somebody else can live.
It is my opinion (purely conjecture and colored by my cynical perception of humanity) that people dislike the Tuvix decision because Tuvix was a more likable and "better" character than either Tuvok or Neelix, so people wanted that to stay rather than what we had. People didn't want to see an overly logical apparently arrogant and stiff vulcan, nor did they want to see an annoying little overly talkative elf with seemingly no manners. We much prefer the calm friendly cool easy to be around much more human (or how we like to think of ourselves anyhow) person we got with Tuvix.
Tuvix IS/WAS "Greater Than The Sum of His Parts"
Another example of TREK Playing it SAFE