Sanji asking Zoro to stop him if he would lose controll just shows how much they respect each other and understand one another. Both would die for the crew and Sanji deciding to die without hesitation, when it would mean to protect his crew and allies highlights that once again! Since both have such a strong feeling of responsibility to protect their crew no matter what, makes it very clear why Sanji would ask Zoro, since he would be the one understanding Sanji's decission better than anyone.