Limitations

Wow,  I can't believe the amount of Schadenfreude all of you have.  It disgusts me.

This was one of the most heartbreaking scenes of all of Code Geass (and those that have seen the entire series know that is saying something).  Nunnally was desperately trying to stop Lelouch from using the Damocles, not knowing that he planned to do pretty much the same thing as her and make himself the object of the entire world's hatred, like she was planning to do with the Damocles, so that he could make the world a gentler place for his beloved sister.

And she didn't learn of his plan until it was far too late.  Watching Nunnally fall was one of the saddest moments of the series, especially since Lelouch needed to make Nunnally hate him for his plan to work.  It was so heartbreaking seeing Lelouch, the one who would find a way to kill any and all Gods if it would make his beloved sister smile, be unable to do anything to help Nunnaly when she fell.  It was even more heartbreaking to here Nunnally denounce Lelouch as a demon, when all he did, he did for her so she could live in a gentler world.