Lelouch doesn't Geass people if he doesn't have to. Also, he tries to use things to their full potential. I'm sure if he didn't have to go against VV, with Rolo a key part of it, he would've killed him right after he confessed about Shirley. He did try to kill him to stop VV after all, he just missed the chance. It worked for him in the end.
(no subject)