...all of this was explained in the last few episodes. Zero Requiem came about largely due to Lelouch's guilt over his "mistakes" - mainly, things like Euphie's death, Shirley's death, Nunnally's supposed death, etc. None of these were his fault, but he still blamed himself. And, by that point, his life basically had no value to him. He'd lost everything. Euphie died, Shirley died, Nunnally supposedly died, the Black Knights betrayed him, and everything he'd fought for with regard to avenging his mother's death turned out to be a lie. (Ironic, considering that Charles and V.V. - and Marianne, by extension - were trying to create a world without lies.) So the least he could do was go out with a bang and at least try to make the world a better place for the few he cared about who were still