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.

I think part of the reason why it feels so much like Janeway murdered Tuvix is because of how suddenly it happens. Sorry, no wrestling with the moral dilemma, we're out of our allocated 40 minutes plus commercials.

I think they could have totally made an episode that argues killing Tuvix was just (it's not like star trek has never had it's captains due morally questionable things that the audience is supposed to wrestle with, looking at you Sisko). Instead, we just got a credits roll.