I know I'm a bit late, but I have to remind everyone that the thing about Lelouch truly considering Rolo his brother is confirmed
by word of God. He wasn't just lying out of pity.
There's also the Hikari story, where Lelouch imagines Rolo as his brother together with Nunnally. I know some people don't consider Talkin' Rebellion canon, but it's included in The Complete, which seems more official than other books to me... And also it has a sort of lulushirley ending so... Shirley fans (myself included) would prefer to consider it canon xD
Anyway, since I love both Shirley and Rolo so much, I blame her death on the circumstances
. Everybody accuses Rolo of killing her out of jealousy, but nobody seems to remember that it was Lelouch's manipulation that made that jealousy grow so intense to begin with. I don't think Rolo would have seen Shirley as a threat had Lelouch not gone "I LOVE YOU NUNNALLY" in front of him (while Rolo was stopping his heart for his sake, too! And he doesn't even receive any thanks either), then say Nunnally's name during his sleep, try to get her back, choose just the opposite when Rolo begs him to stop being Zero, go to China and leave Rolo alone, then leave him alone again thanks to Sayoko's awesome 6 months-long list of dates schedule (with only 3 hours to sleep I doubt he had any time left to spend with Rolo), etc.
Rolo must have been very desperately craving for Lelouch's attention at the time Turn 13 took place, and...
I know, Rolo could have accepted Nunnally as a sister, but Lelouch ignoring him so much after around Turn 5 made it clear to him that he was going to be abandoned when Nunnally came back. He states it on another story from Talkin' Rebellion, that he's afraid about what might happen to him if Nunnally ever came back.
So yeah, I think Rolo, Lelouch and the Cult should share the blame among the three... plus the staff to for treating Shirley like she doesn't even deserve to appear on the character chart ŽA`;; At least she got a bigger role than Rolo on the Zero Requiem Special Edition. Still, see? Rolo was there too as the other most important trigger to Lelouch's actions. Sunrise wants us to like Shirley and Rolo both so... let's not fight with each other
*forgive my bad English*