Three suggestions .
some villain he left alive, go back and kill someone close to him, threat his family or anything else.
Ot how about, the righteous character, lose his faith on everything he used to believe at, and seeing that being righteous is nothing more than a bull-shit..
The righteous character find something he wants so badly, it can be woman, peace, or even something so righteous. but to achieve that thing, the character would have to go against its own moral.