The Borg are just acting with the will of the collective, so is Tuvix, neither feels the individuals have any say in the matter. Why is taking one apart OK and not the other.
Tuvix was not an imminent threat to the ship. The Borg are always a threat (collectively speaking).
So what? That doesn't answer why the Borg are less deserving of survival than Tuvix.
Like I said, the Borg are always a threat. The Collective's entire existence is driven by the threat to assimilate others. It cannot, by definition, be permitted to exist. That's the difference.