For what we know now is, that he used a method to surpress Kishins Madness that force him to stay in Shibusen that was so grave, that he still can not leave the city.
What i am wondering is, why couldnt he just kill him? Why did he need to take such measures?
Is this the reason he founded Death City and the Weapon/Meister System, because he couldnt leave the city anymore?
Or is it a reason we still dont know? Maybe thats why he needs the demon tools
