Killing two people who are both destined to die in the very near future to save one who will live for a considerably longer period and save a greater number of lives would be the right thing to do from a utilitarian standpoint. Tuvix, meanwhile, is a healthy being who was competent to discharge his duties and posed no threat.
Consequently, while there is an argument to be made that killing one person to revive two others is a net benefit, the burden of suffering on that one person is extreme, and whether or not it is outweighed by the positive nature of the two others returning to live is very much a matter of individual outlook.
It is also worth noting that Tuvok and Neelix as they existed before could be considered "already dead" as a result of their combination into a single entity. Thus you could argue that what actually happened is that Tuvix "died" so that clones of the deceased Tuvok and Neelix could be created from him. Admittedly this is a shaky argument given the whole "do transporters actually kill people in-universe" thing.