from post # 24, Mecha Ethics: Tens or hundreds or thousands of Britannian soldiers obeyed Euphemia's orders to kill Japanese at the Fuji stadium. Unless you have proof that Lelouch knew that Euphemia was the last Britannian left at Fuji then he should have captured her and used her as a hostage to stop the massacre instead of killing her.
Armchair general; this reminds me of a case when a police officer in Spain, or maybe it was Portugal, was in a hearing before judges because when he was off-duty he had killed an armed, drugged-up murderer in the street by shooting him from behind. One of the judges asked if he couldn't just sneak up and knock out the man. The police officer said that's what people imagine when they have been watching too much TV.