I had long been in the "killing Tuvix was beyond the pale" camp, but my Brother made an arguement that got me reconsidering my position.
It was when I read an article that compared it to organ transplant: what if you had to kill Tuvix to harvest his organs and give them to Tuvok and Neelix.
That's fairly Morality 101 (in fact, I have seen it used in a show that is a professor's lectures on the subject): you don't get to kill someone, even if their parts would save multiple lives.
My brother pointed out how this one is different: those parts originally belonged to Tuvok and Neelix.
We can't force you to give up your kidney to save the life of another. That would be immoral: it is your kidney, and it is entirely your choice.
If somebody takes your kidney by force, you have the right to have it back.
Now here's the real crux: even if it has been implanted in someone else. It is still your property, since it was taken without your permission, and you should get it back if that is what you want. Even if removing it will kill the other person.
Now, if the other person was blameless in all of this, we do kinda hope you are a nice guy who will let him keep your kidney, but you are under no moral obligation to do so. We cannot make the theft legitimate just by going, "Oop, gave it to someone who needs it, so now you can't have it back."
Tuvok and Neelix were unable to give their permission. It is possible that they would have chosen to allow Tuvix to keep their parts, but we have to ask them. If we can't ask them for some reason (under anesthesia, perhaps), we have to assume they would rather live than give up their parts to save another.
My brother also uses a very strict definition of "dead": if you can come back, you aren't dead. Thus, Tuvok and Neelix were not "dead" yet. If they were dead, their rights to their organs would be reduced (to nothing, in fact) and we should let the live guy keep them, but they aren't dead, and failure to return their organs to them would be killing them.
Because he can only live by harming Tuvok and Neelix, Tuvix can only live if they both consent to it. We cannot compel someone to support another person at the expense of their health or life. We can applaud those who volunteer for such support, but we cannot compel it.