The question to me isn’t whether Janeway murder Tuvix, but was the murder of Tuvix justifiable. In Star Trek 2 Spock famously states “the needs of the many outweigh the needs of the few” in TNG Thine Own Self Troy learns that sometimes an officer must order a crew member into a situation where they know that person isn’t coming back.
Does the situation Voyager was in and the creation of Tuvix represent the same level of danger “to the many” that say an imminent warp core breach does?