You bring up a good point that I forgot about. Lelouch was disturbed by what he had done to Clovis, and that could have been what stayed his hand with Euphemia. I doubt Lelouch ever point-blank killed someone before, and with Clovis, he was jacked up on his vengeance kick since they were talking about his family and all. With Euphemia, things were more passive and relaxed and I think subconsciously Lelouch rationalized and chickened out. I do wish they would reference this again. Maybe during the island arc.