@GrayBypasser
well from looking at the first chapter.
His students were taken hostage and they used a child to kill him.
So I'm guess they used the hostages to restrict him and then the child to stop him from resisting death when it came for him. Perhaps they knew he had a soft spot for kids.
It's pretty screwed up way to kill a person to be honest.
but then when you think about it, how else would they have done? Look at that last fight he had with the demon. They used his weakness for his students and children because attacking him from where is strong wouldn't work.