When it comes down to it, Victor's creation is an abomination. It goes against all teachings of that time period. Men aren't meant to play God and create life from something inanimate or dead. To reveal his creation is to reveal his obsession and that is something Victor can't abide. This is mainly because Victor knows he shouldn't have made the Creature. This is shown in his immediate reaction to the Creature when he animates it: he runs from the room at the realization of the monster he has created. So, to reveal what he knows would have saved Justine, but it would have been at his own expense and that was something Victor couldn't do.