I've read this Tuvix argument and watched it go on for years and I find that people miss the point entirely. Janeway did not murder Tuvix. She did what any command bridge officer is supposed to do in emergency situations. Voyager is thousands of light years from home, with no chance, with the exception of the occasional stray along the way like Seven and Neelix, to have the ship restaffed with qualified personnel at a Starbase.
In the TNG episode Thine Own Self, Deanna Troi had to learn that to command a starship, she may have to order a friend, or a member of the crew, to sacrifice themselves for the safety and the well being of the ship she commands. Janeway liked Tuvix. But when there was a way for her tactical AND second officer to be restored, the command descision to make would be to order Tuvix to be separated so Tuvok could be restored.
It may seem ugly, or immoral, or even murder to some. But if I were captain and in her situation, you better damn well bet I would order Tuvix to undergo the procedure. My duty to the ship and it's crew comes before personal or moral implications. Voyager needed it's tactical, security and second officer restored.
The needs of the many in this case, Voyager and it's crew, outweighed the needs of the one, Tuvix.