Zhang Yi was shocked by the atrocities he saw Oz'Gorath do in his millions of years of his life and also because Oz'Gorath did it all, but after recovering Zhang Yi decided that he would deal with that information after destroying the demon universe.
Zhang Yi used his clone technique for the first time after arriving in this universe and so 12 clones appeared, his technique had strengthened again after Zhang Yi reached the Eternal Realm and even though his clones had only 10 percent strength of him, today it would do to destroy a universe like this.
Zhang Yi's clones did not have to destroy the smaller worlds as Zhang Yi did because he wanted to ensure that he would kill Oz'Gorath completely, but all demons had to be exterminated.