Except in Light's case, it's not even a case of revenge. It's nothing more than pure self-righteousness. He found some power and his thought was to, in his mind, cleanse the world. Unlike with Akechi who you could say was manipulated and pushed into doing what he did by Yaldabaoth, Shido, and Society as a whole and knew full well what he was doing was evil, Light had absolutely none of that.