We learned earlier on that Ki is something you can only have 1 type of in the body, if you have 2 or more it disrupts your body and the Ki that you are working on. Metal Ki might be similiar to what Noh's master was going to teach him, but its not the same.
He has been raised/trained like a (insert reference) for years so that his body would be ready for the final step, only for him to mess it up and almost degrade it to a point of no-return.
Yes, morally how they are doing it is wrong, but words alone is not enough for Noh at this point because he is becoming delusional with his new found strenght, they are only trying to make him realize that his new power is useless in comparison to what he was supposed to learn, and by the end of it he would probably be extremely thankful to the elders for beating some sense into him.