Tuvok and Neelix were dead. A tragic accident, yes, but dead.
They were obviously both trapped inside of Tuvix as a result of a transporter malfunction. In Tuvix they were merged accidentally but they never stopped to exist, Tuvix was made out of them.
Tuvix was a living being who wanted - and had a right - to live. How he came to exist is irrelevant.
True, however - that did not diminish Tuvok´s and Neelix´s right to exist as individuals and as we know 2>1. The right to exist of two naturally born and socialized beings clearly outweights the right of one individual who came into sudden existence by splicing the two through a technological malfunction.
Janeway murdered an innocent living man to bring two dead man back to life.
No, they where never dead, they where just merged and luckily that could be corrected - for a high price but it was worth it. Let´s not omit that Tuvix was no natural being, he was never born, had no parents, no family, no history and no social life prior to the accident. If you think about it you have to admit that everything that made Tuvix - his genes, his face, his feelings and memories, everything - was not his but belonged to Tuvok and Neelix because it had been taken from them to create Tuvix. Everything Tuvix was belonged to Tuvok and Neelix and as the original owners they had a right to get back what had always been theirs.