Shen Yu was mentally exhausted after all the work. Although, he was affected by the destruction of whole Sect, he was more affected by the sight he has witnessed there.
After all, the disciples did deserve to die since they had attacked Lan Ming with the intention of killing and even injured her so much. How could Shen Yu let them go? The most he could do to them was give a traumatic death that they would remember in their best life.
However, the sight of Lan Ming covered in blood from head to toe was something he hoped he would never again. In fact, he was already making plans to keep her wives safe at all cost.
Even the necklace plan was good, however it wasn't that effective. There were only two ways in it in which his consciousness would materialize.
The first one was obviously when Lan Ming or any of his wives called for his help. And second one was if Lan Ming was dying. Since he didn't want to hinder their progress, he had kept the requirement limited to death.
