IHe wasn't killed, he just ceased to exist, just like how he wasn't born either......
Actually, he was born. In the episode, it was clearly stated that Tuvix was the result of symbiogenesis--which is a form of reproduction.
Only with the plant they were transporting. Tuvok and Neelix themselves could not have parented Tuvix, so in that sense, he was not born.
And as I keep pointing out: Why does Tuvix's right to existence OUTWEIGH that of Tuvok and Neelix? Why does the need of ONE take precedence over that of TWO?
Think of it this way. If Tuvix is indeed a 'child' of Tuvok and Neelix, then in a very real sense, Tuvix's birth would mean their deaths. I think most of us, whether we be pro- or anti-abortion, would grant that if a baby's birth would mean the mother's death, that would be an argument for terminating the pregnancy, wouldn't it? So why would one not allow that baby to live, but insist that Tuvix live? The principle is the same: The existence of the child cannot come without the death of a parent (in this case, both 'parents').