how would rc have something to do with the attack? he may have shown signs of being an 'evil nobleman' at that age, but why would he assist in the killing of his family (or just taking them down, if killing them all wasn't part of the original plan)? he was clearly going to succeed vincent as earl anyway, and he must've known it would upset ciel. i'm not hating on the theory; i'm curious and i want to understand it!
Don’t worry, I definitely understand where you are coming from! As of now the RC mastermind theory is only based on circumstancial evidence and lot of assumptions/headcanons, so it’s not as solid and comprehensive as some other theories we had in the past, like the 2CT which had hard evidence (both verbal and visual)!!
Anyway, the two possible explanations
I’ve seen often mentioned in the JP fandom
for why RC might have been involved in the attack are
1) “It’s a ritual”: killing the predecessor is obligatory to succeed the Watchdog title (→flaw: RC just turned 10 that day and that would be waaay too early to succeed a tough job like the Queen’s Watchdog)
2) “RC is a super-yandere/psychopath”: RC didn’t want to become the Earl and be separated from OC, wanted to be the only one close to his little brother and therefore tried to get rid of everyone else (→flaw: if he hated to become the Earl so much, why would he protect the Phantomhive ring by even going to the trouble of swallowing it?)
As you can see, both explanations have flaws and are not really convincing, and that’s exactly why I, too, still have doubts about the RC mastermind theory!
But then there’s this scene from Murder arc:
“I had no reason to murder him”
“The reason for which one person murders another is typically inconceivable to other folk. People will never be able to understand another’s mind, regardless of how much research genius schollars collect on the subject.”
If this is supposed to be some sort of foreshadowing, then maybe the reason why RC might have had something to do with the incident (or killed his parents/stabbed Tanaka if you go with the psychopath theory) is something very unexpected
Anyway, this theory has been long around in the JP fandom since about the same time as 2CT (i.e. since 2011), and the fact that a lot of people are still suspicious of RC even after 6 years makes me almost believe that Yana wants us readers to suspect RC. I think she’s placing the hints on purpose, but it’s just that we don’t know whether these hints are indeed leading us to the true culprit of the attack OR they are a red herring and Yana’s just misleading us into believing RC is the culprit. For now, I think both are totally possible! :D
Vincent’s line in the special chapter
“Love is a wonderful thing… but it can aslo give birth to horrific disaster.”
might also be a foreshadowing of why the attack on the Phantomhives happened that day, but idk.
Yana loves foreshadowing so every random panel or line could be interpreted as a foreshadowing of something xD