"I agree to do this for what you have done for my sect and because I agree that perhaps only I will be able to kill this demon, if I do not do so it may be that this demon will be set free and will eventually destroy this continent and my sect as well."
Zhang Yi did not say that but thought that if he did not do so he might find it difficult to cultivate in the future and perhaps even have a demon in his martial heart for running away from doing something right that was surely his duty to do.
Meng Wei and the island leader were impressed that Zhang Yi made such a decision so quickly, after all, even if he had the ability to do so, there would be few who would have the courage to do something like that.
Mainly because they both knew how horrible the many years Zhang Yi had spent in the underworld must have been, he must have nearly died dozens of times and it took a long time to destroy that underworld.