Napoli was fairly simple to turn evil. I simply granted one of his deeply ingrained wishes. Current Napoli thinks his life would have been so much better had one thing in his life have been different but as it turns out, his life would have been a lot worse. Of course, I also added some extra edge (gotta keep up with Gratia) and took some cues from Char Clones. 

Kiran on the other hand was a random concept that I'd had ages ago, namely pushing the idea of him respecting Grimm even further and see how that would have happened. As it turned out, he became a wild animal.