Tuvix was made of both Neelix and Tuvok, and to keep him alive would be to condemn two officers to certain death. Why should Janeway allow that if there is a chance to save them? Yes Tuvix is a life form himself, but so are Neelix and Tuvok.
Neelix and Tuvok were already dead. They were lost in a transporter accident as has happened to many Starfleet officers.
Janeway gave up that one alien made up of the body of a dead crewman who still had her memories. So did Janeway have a double standard in her case? Should Janeway have fought to keep that one woman aboard and kept trying to restore her humanity?