Board Thread:Naruto News/@comment-46383374-20200805201515/@comment-32685487-20200805231338

He said to Madara at their final battle that he would kill even his own child for the sake of Konoha and it's peace. We never see Tsunade's parents and it was heavily implied all she had left was her and Nawaki, then Nawaki died and she had nobody left. So MAYBE Hashirama did exactly as he said he would and killed his child at some point.