An Zhichen doesn't forbid Tingzheng from going to the Imperial Guard Battalion out of revenge, but because he no longer trusts Tingzheng. Now that the Ninth Prince is the Crown Prince, the surface of the Capital City appears tranquil, but the Seventh Prince is still in the Capital, and the Emperor hasn't eliminated his influence. Beneath the surface, the Capital is still turbulent. With Tingzheng's character and Lin's impulsive nature, if Tingzheng is left in the Imperial Guard Battalion, who knows what trouble might arise in the future. If Tingzheng causes trouble, it will definitely implicate him.
An Zhichen is not like Yueyao, knowingly supporting him even when it's wrong. Since it's known that Tingzheng is unreliable, naturally it must be quelled in the cradle.
When the people of the Lin Family learned that Tingzheng had a new position, Lord Lin sighed, while Lady Lin was deeply worried.
