The writers.
Seriously, I think one of the reasons that episode still inspires arguments almost 30 years later is because the writers "cheated", in multiple ways. Some of the most important things one would need to consider to arrive at a moral/ethical decision in the matter are either invalidated by the premise (smashing two consciousnesses together makes a third one??? with no complications???) or hidden off-screen (do Tuvok and Neelix remember being Tuvix? how do they feel about what happened? sorry, episode's over).
It pains me to see people trying to puzzle out an answer by analogizing with abortion, or euthanasia, which are already fraught with enough argumentative weight without trying to balance a not-very-good episode of Star Trek on them.
Let's just acknowledge the loss of what would've been the truly cool option: decide that this is one of the episodes where we're not going to worry about the implications of transporter technology, and bring back Tuvok and Neelix while keeping Tuvix around. "But (sigh) that would've been interesting, and we couldn't have that," says the Voyager fan.
