So he was wrong to deny Tuvoc and Neelix's right to exist, but Janeway was right to deny Tuvix's right to exist?
That's the problem. From a moral and ethical standpoint, none of the options are firmly right or wrong. I can't make the choice simply on those standards, and I don't think Janeway or anyone else could either. From a pragmatic standpoint, I can see some justification for either option.
This.
Janeway made a choice as captain. She picked two lives over one. To me, it's really that simple.