The way I see it, Tuvok and Neelix died in an accident, and a separate life emerged from it. The crew just couldn’t accept their deaths so they killed Tuvix to get them back.

I don’t see it as a logical decision but an emotional one. How Tuvix came to exist doesn’t matter, he was still a person, and they basically murdered him to get their friends back. He wouldn’t be the first living thing that wasn’t “meant” to exist.

Either way it’s a very difficult moral question and probably the best episode in Voyager as far as emotional impact.