Rosatrice is a fan created term to refer to an alternative explanation of the murders in Umineko in which Rosa Ushiromiya is the mastermind behind the incident, with George Ushiromiya and Nanjo as main and only accomplices. It is based on the notion that Rosa's lack of family love, romantic love and guilt for the death of Kuwadorian Beatrice, as well as interest in the occult, led to the creation of the imaginary furniture servant Yasu who later became Beatrice (both being roles she played), and planned murders according to the epitaph in a desperate attempt to regain love through the resurrection of Kuwadorian Beatrice. Another version is to chose pure-hearted, Kinzo - equal head of family, as all other has their sins, even Rosa herself, and they don't have enough power to handle wealth of family in good way.
It has no official support besides evidence gathered by it's believers from the novels, and since the theory also states that Shannon and Kanon are not directly related to Yasu, it is in opposition to the belief that Sayo Yasuda is the murderer in Rokkenjima.
It is highly advised for the reader to make their own conclusion.
The theory uses both arguments in favor of Beatrice being a creation of Rosa and arguments in favor of Shannon and Kanon being different individuals disproving the arguments for them being same person. It's main premise is that Yasu is intentionally depicted as being the murderer through it's different personalities, and is depicted in this way as a red herring to hide it's relationship to Rosa, the latter being the true mastermind culprit of Umineko.
The theory is criticized for not being openly supported by Ryukishi07 and for its premises relying on specific interpretations of red statements, not compatible with the overall plot (need evidence of contradiction to overall plot, otherwise this is not encyclopedic, or reliable data, same with statement on Beatrice page (her "real" personality). Furthermore, in 2015 Ryukishi confirmed in an interview that the manga contained the official answer. (Though the official answer is not the real one, for the Rosatrice believers.)
https://www.youtube.com/playlist?list=PL48C47AC36EAD9A04--- Comprehensive explanation of Rosatrice theory.
First twilight Edit
Rosa and Shannon's death were faked using the fake death drug, although their wounds weren't faked at all. (Considering Shannon may have or may not have accepted the fact that George was a murderer, he probably wouldn't had taken the risk of telling her the truth, therefore, used the fake death drug without her knowing it.)
In the morning, when the corpse were discovered, Nanjo lied about Rosa being dead, and the objective point of view of Battler barely talk about Rosa.
As the plan of Rosa was to kill everybody, it can be possible to say that Rosa had killed Shannon when she was in the shed. As a murderer, Rosa may very well have a second plan, this plan would be to betray George. With that betrayal, there is only one corpse faked, therefore, it correspond to Will solution.
If George had seen the corpse of Shannon, he would have known that Rosa had betrayed him, therefore, Rosa took tremendous risk.
Another way for Rosa to kill Shannon is to have put real poison in the fake death drug poison, so that George would have killed her without knowing it.
It is very much possible for Rosa to have escaped the shed after everyone was gone, the shutter can be destroyed using a tool, and she could have slipped through the window. (We don't know how small it is, but in the anime version, we can see Battler next to the window, and he could have slipped through it.)
Also, Genji may have pretended to lock the shutter, on Rosa's order, that way, there are 3 possibilities to get away from that shed.
Second twilight Edit
George went to his parent's room and asked them to let him in. Given that it was their son whom they trusted, Eva had no problem letting him in while Hideyoshi was taking a bath. George then quicky killed both.
Rosa was hiding in the mansion then met George in a prearranged location. Rosa then entered the room, and planted the stakes in the corpses. She then set the chain and put the letter under the door. She hid herself under the bed, waiting for someone to come.
When Genji and Kanon have realized that the room was chained, and were looking for something to cut it, Rosa went unset the chain, go outside, paint the magic circle, and then hid herself again (And of course put the chain lock on again)
In the anime version, George jump on Eva's bed to hug her, pretending to be sad. This was a smart move of him, as nobody would check under the bed to see if there was someone, considering you would just let someone who lost his mother alone, without annoying him.
We don't see what happens for several hours, therefore, George could have possibly killed his parents, it is even mentioned in the VN's that a lot of people left the parlor and came back again.
Another solution is that Rosa was hiding in the guest room even before Eva and Hideyoshi came in. When they weren't expecting anything, Rosa jump out and killed everybody, then hid herself again.
Fifth twilight Edit
Kanon was not killed in the first place, this is confirmed in the manga version.
In fact, Kanon faked his death himself in the boiler room, even though he is neither the culprit nor an accomplice.
In the Rosatrice theory, Kanon may (or may not) be aware of what is going on on Rokkenjima, so, to go against Rosa-Beatrice, he decided to be the "0 on your roulette" (Anime and VN version) and to bring down Rosa's ceremony, therefore, taking his fate into his own hands and faking his "suicide" in order to fake the fifth twilight, in simple words, to "avoid being killed".
He was sedated by Nanjo when he was found.
As Kanon has served Kinzo for a year now, he would have known where the stakes were, and so got his hands on one of them.
From sixth to eighth twilight Edit
Rosa stepped into the parlor holding a gun, she asked Maria to face the wall and sing a song. She then shot Kumasawa, Nanjo and Genji. Right after that, she dialed the number of Kinzo's study, left the phone of the hook, left the parlor, and locked it with Shannon's or Gohda's Masterkey.
Ninth twilight Edit
In the red regarding the boiler room, the dead are given an alibi as well, suggesting that "There are no unidentified corpses, and all of the survivors have alibis!" is a problem of terminology, Namely, the people who are believed to be alive and the people who are believed to be dead.
Another possibility is that Kanon woke up, got his hands on a gun, and was looking for Rosa (in order to kill her). He then bumped into Natsuhi, who thought that Kanon was the culprit. They then shot each other, directly killing Natsuhi.
Of all the Rosatrice believers, there is an overwhelming majority of people who actually thinks that Kanon is indeed the one killing Natsuhi, or that she killed herself. the reason is that, it is very well possible for Rosa to have killed Natsuhi, but Rosa had to be alive after Natsuhi's death because she appeared just seconds before the bomb went off to the survivors (In the VN)
The letter is not a problem in the ninth twilight, there may never have been a letter to begin with, and there are several ways that the letter may have disappeared.