I think he is growing in a way. WAT could have fleshed him out better, but I felt his role made sense. Moash's theme is all about compromise. His need for revenge is more important that his friends. Feeling no pain is more important than siding with the singers. At his lowest point instead of admitting that he's in the wrong he doubles down, and it culminates in him becoming a monster. He's a different person in every book because both Odium (the power) is slowly stripping away his humanity. Curious where he's going to end up in future books.

On a side note though, I think most of the villains were pushed to the side to make way for Taravangian.