Why did Lelouch continue to play the villain for the sake of a gentle world?

Contains spoilers up to the ending of the TV anime series 'Code Geass: Lelouch of the Rebellion' and 'Code Geass: Lelouch of ...

After finishing 'Code Geass: Rozé of the Recapture'. I thought the 'Code Geass' I knew had returned, but in the end, I saw a slightly different landscape

Rozé of the Recapture', the first thing I thought was,'This is exactly the kind of series Code Geass is, isn't it?'That was ...