Hm. He had an opportunity to let the Sword Saint kill everyone else and find the traitor. Also, if he became the traitor himself, he would have had an opportunity to get himself killed by the strongest people and gain their skills - including perhaps the lie detection ability. He could have done all this and returned to save the lives of the innocents. It would be tedious to do it over and over again, but it would be beneficial.
I guess that would make things too easy though.