Janeway had to
separate Tuvok & Neelix and therefore kill Tuvix, because she knew she'd already done it
vision, during the fight with the Vidians, of a future TUVOK
walking into a cargobay full of borg children
told her it had been done, so naturally that's what she did.
[Hated season 5's "Fury" reference... I'm too tired (long week) to read the next 9 of (currently) 12 pages to see if someone else already mentioned this idea. If so, then I defer to their right to crow.