No-face saw how the bathouse workers reacted to the gold from the river spirit. Since he was lonely he thought if he made some gold for them, they’d love him. And he was right. Of course, he couldn’t really make gold–just fake gold. Just like he helped Sen by getting her the bath token she needed. Then he figured she wanted more bath tokens, and was hurt when she didn’t want them.
He couldn’t talk until he ate the talking frog. Then he could talk using the frog’s voice.
So, when he started giving the workers gold, they reacted with greed, and he began to act like they did–consuming more food, more pleasures, until finally he ate the two people. He became out of control because he didn’t understand how to react. He wanted to be like everyone else, but had no limit to his behavior. Everyone was encouraging his gluttony, so he became gluttonous, until he went too far by eating people and destroying things.
Then Sen finally gave him the River Spirit’s medicine, and he regurgitated everything he had absorbed and experienced at the bathouse, and returned to the gentle spirit he had been in the beginning. And Sen leaves him with Zeniba, because No-face needs a gentle quiet place to learn how to be a person, if he is around greed and violence he will become greedy and violent himself because he doesn’t know how to act.
So No-face’s story is a kind of reflection of Chihiro’s story, except he didn’t learn and grow from his experiences at the bathhouse like she did. Instead he turns into a monster, forgetting his true self like so many of the other spirits there. Chihiro saves him from this and takes him away to a safer place for him.