Lelouch has already killed Carmen Sandiego since he got here, man. And no one ever found out. He can also take control of the mind of anyone who is looking him in the eye. I don't mean to tell you Sarah isn't awesome, but adding to the pile of pre-existing reasons why Lelouch dislikes Chuck is really, really not recommended, even for her.
(She's not armed yet, by the way. I'm sure that'll last ten minutes, but Acumen takes her weapons. In case you didn't know!)