Narukami went with it as he accepted Zero's request. No harm either, only benefits. He needed as many Sorcerers to reveal the truth about All Beyond.
When that happened, All Beyond would undoubtedly deploy all of its resources to kill him.
Although dead come to everyone. That did not mean one would wait for it should it be before them. One might struggle and use every last strand of energy to save their lives.
No exceptions to Narukami. He knew what might happen to him in the near future and so, doing this would benefit him a lot.
Was he hesitant? Sort of. It was because Zero was one of the upper echelons. There was a high chance he was acting but in truth digging more information about Narukami
Nevertheless, Narukami had many ways if Zero tried to black mailing or doing something which disrupt or in the way, Narukami would not hesitate to kill him.
He was not being cruel. If not for Zero's usefulness, Narukami might have killed him already.
