What do you mean that the real Ciel is yandere ??
‘Yandere’ is a Japanese term for a manga/anime character who is obsessed with someone in an unhealthy, twisted way and I thought this term might fit RC because his words towards OC sometimes sound typically yandere-ish, i.e. very possessive and controlling! xD
For example, there’s this line from the side story “That butler, nursing”
which you can interpret as
“I don’t need fake brothers (because I already have my one and only twin brother who can’t be replaced by anyone else)”.
his words in the dream sequence in ch95
“I know EVERYTHING about you.” (very typically yandere)
this line from ch129
“I will never leave you alone again.” (very typically yandere)
and his reaction in ch132 upon hearing that he might be separated from OC one day
“We can stay together forever.”
“Why do you leave me alone?”
Another common trait of a ‘yandere’ character is that they can be very violent and murderous, and are ready to do cruel crazy things if someone gets in the way, which is literally what we’ve seen in ch126-128!
So considering RC’s action, behaviour and words so far, I really don’t think this “RC is yandere theory” is that far-fetched! (whether people like this plot twist or not is a different matter)