suzaku is lawful good. he abides by a predetermined set of principles in hopes of achieving peace (for nunally, euphie, whatever).
lelouche is lawful neutral- he abides by a predetermined set of principles, but doesn't care about the result- as long as it means crushing brittania.
just because your actions have evil results or because you work for an evil empire doesn't mean you're evil. it's the intent that counts (according to gary gygax at least).