If Tuvix really had the qualities of Tuvok and Neelix, he would have freely volunteered his life to save both Tuvok and Neelix. That he was unwilling to do so demonstrated, concretely, that he was as a mythological changeling, as a troll exchanged for a human infant, but, in this case, two. For all intents and purposes, he was a manifestation of the abduction of two crew members, lacking the character of either. Additionally, as the degradation that he was, in the long run he was even potentially a security threat to the ship. Good riddance.