If there was sympathy, they wouldn't have let Kathy Kill him.
Even if the entire crew said "let him live" it was till supposedly
the Captain's call... For as long as as they were all on Federation soil. If Tuvix got off the ship, Kathryn lost all her legal powers as a Starfleet Captain.
What if Neelix said a magic word like "Asylum"?
Would Janeway protect Tuvix from Janeway?
Would Janeway be of two minds about how to treat a man of two minds?
You can't keep a compelling story without hard choices.
One choice is no choice.