Although he was no longer in command of the city, he was a man who, at one point, was one of the greatest powers in the world. And his strength could not just vanish, so he was still a great power.
If he were the one to do it, the Adamses wouldn't worry. Once the entire Kaze hereditary line was eradicated, they had more freedom of movement and could initiate a large-scale battle to destroy that Clan once and for all, even if the 3 Leaders were involved.
Naturally, they had confidence against the 3 Leaders, and it was not due to themselves but to the Old Leader. If he promised to help them, they would both have an alliance that would resolve, in the meantime, the Leaders, for the greatest hatred in the heart of the Old Leader is those three, and those three have long wanted to kill him.
Basically, Adams wanted to use the Old Leader as an assassin against the entire younger generation of the Kaze and, in turn, as a shield to give him a chance for revenge.
