So if this was all because Kes was crying what would have happened if Kes had liked Tuvix better?
"Captain everything that was ever missing in my love for Neelix is now embodied in Tuvix, this is the happiest I have ever been!"
All Kes crying over wanting Neelix back did was just demonstrate Janeway had a preset disposition against Tuvix. She was critical of him because he was so desperate for his life, begging already having failed he sent Kes to beg on his behalf.
It's a non-factor. Janeway had already decided it was within her right to play god with his life. That's the root of her wrong. If Kes had spoken on Tuvix's behalf, it's debatable whether Janeway would have gone through with it or not. This is really a situation where one person standing up for him may have made a difference. One person saying it was wrong. Well the Doctor did, but Janeway is on record saying he isn't a person.