It has nothing to do with the cause of the accident and everything to do with who "owns" his own body. I agree that Tuvix didn't cause the accident, but just because the accident happened is not reason enough to commandeer another's life. Tuvok and Neelix had "title" to their own bodies, it doesn't matter if Tuvix was sentient, it doesn't matter that he was an individual or that he had feelings. He had no right to take what wasn't his to begin with. I can and do deny Tuvix "his" so called life because your argument can apply just as easily to Tuvok and Neelix and they have an established prior claim. They too are sentient, individuals and they too have a consciousness.