Shadow Dao was a two-edged Great Dao with a critical flaw that was very easy for evil people to exploit. However, Wang Ling felt that this flaw could be fixed. After all, Ah Nuan had yet to be born. She was probably the most powerful embryo in the world, or even the universe, right now, but she still had limited control over Shadow Dao. As long as Ah Nuan grew up and got rid of the aspect of Shadow Dao which triggered "evil," it would still be a Dao that would benefit all of mankind.
Wang Ling couldn't say for sure if Wang Nuan would cause all sorts of havoc after birth like he had due to lack of control over her power. What he could be sure of, however, was that Father and Mother Wang's instructions for him hadn't changed: As her big brother who had experienced the pain of losing control before, what he could do was help lead Ah Nuan on the right path.