I always come back to that scene with Suzaku in the student council room, early on, when he’s still keeping up the pretense. He says something like, 'The only ones who should kill are those who are prepared to be killed.' At the time it sounds like detached philosophy, but it’s the core of his entire deal. He wasn’t just talking about physical death; he was accepting the death of his own morality, his own chance at a normal life. He was prepared to be killed as Lelouch Lamperouge, the nice student, so that he could become the monster who would burn the world down to save his sister and create a gentler one.
It’s easy to point to the big, dramatic proclamations later—'I destroy worlds and create new ones'—but the quieter admissions are more telling. His rant to the heavens after Euphy’s death, 'If power is justice, then is powerlessness a sin?' That’s raw, unvarnished rage at the system he’s vowed to break. It’s not just about Nunnally then; it’s a fundamental scream against a world where his family’s might made right, where his powerless mother was killed and his sister used as a pawn. That quote shows his motivation isn’t purely altruistic—it’s fueled by a deep, personal vengeance against the very concept of Britannian 'justice.' He needed to believe his crusade was for a noble cause, but that line betrays the wounded child underneath the mask.