If Light didn't do anything, he would live a normal life as a student etc.
If Lelouch did nothing, him and Nunnaly would still be used as pawns for Brittania (reason they were sent in Japan) and the Ashford family couldn't protect them forever.
Also, he still wants revenge and asnwers for what happened to his mother.
Thus, for freeing Nunally from being a pawn and taking his hate out on Brittania, has reasons to fight against the Emperor.
And he admits that if he has to be evil to accomplish it, let it be so.
Light was just stroking his own dick of becoming god of a new world he'd create, and anyone obstructing his 'justice' was evil.
Still look the exact same to you?