Chapter 666

[JAC]

Maybe she didn't kill Shiki in the ritual. It's possible that she killed everyone who participated in the ritual because Shiki was taken down in front of her and she couldn't do anything about it but this doesn't make sense because there can be only one, at least that's what I think but I could be wrong. This whole ritual thing is kinda pointless because the more soldiers you have the better off you are, I understand you're trying to create a heartless killing machine one without remorse but not everyone is cut out for this sort of thing. 

(Ch.666 Pg.20)

[ashjacob47]

My guess is Rei did kill Shiki but that only happened because Shiki voluntarily allowed herself to be killed by Rei, due to Shiki's feeling prevented her from injuring/killing Rei.

In regard to the ritual, unlike other assassin clans, only one Shiyuu can exist at a time, thus the 'last-man-standing' (or maybe we should call last-girl-standing) in the ritual is the best method. Other members of the Shiyuu clan are simply nameless assassins.

[JAC]

Good guess. Seems like it was never Shiki's intention to hurt Rei and vice versa.