We still don't know anything about Schneizel's background, but if Euphemia wasn't lying when she said that Cornelia had been investigating Marianne's death, what Clovis probably meant back when Lelouch killed him is that Cornelia and Schneizel had been trying to find the culprit as well. Would Schneizel do this just because Cornelia talked him into it? I think not. Maybe Schneizel was also close to the other children. If meijioro's theory is correct, then all the more reason to kill Marianne.

On a side note, I found it pretty interesting that Schneizel's gesture at the end of Stage 18 was the exact same as Lelouch's.