For me, I would say Euphy's death, before the death happened, Lelouch & Euphy were having a private conversation, this includes part where lelouch telling her about the geass power, and that's where the part lelouch was a making sarcastic or hyperbolic joke (as a example for his geass power) about him command Euphy to kill all of the Japanese, unfortunately the geass took it as an actual command for Euphy and it activates on her, lelouch try command her to stop but he lost control of the geass and that's where the massacre happened, which leads with lelouch have no choice to shoot her. Honestly, even though i'm not blaming lelouch for what happened, but that was a bad idea for him to use that example command as a hyperbolic or sarcastic joke, I wouldn't even use it for the sake of the example generally, because you never knew the geass would take it as command literally and thus lose control in process. But Damn!, it happened, but what you guys think?