So in my fantasy novel thing, I plan to have the main character's older brother Jelani fall in with bad guys, so that the next time the main character sees him, his brother is basically a sadistic psycho. Problem is I'm having a hard time figuring a proper way to portray it. Jelani left the village originally because he wants to make a name for himself. He was tired of being a peasant, having to toil away at the ground while the world passes him by, so he leaves to join the rebel army against the Empire. And it's around here where I'm not sure what could make him turn into the complete monster I have him being in further installments. I plan on have him discover his magical abilities like the MC does, and the villain finds him and says he can help him learn to control it. But that's about as far as I've gotten. Any ideas?