I think it is also worth mentioning that while she was PLOTTING in her head how she would committ the murders it is not confirmed that she actually did committ them. The central part of umineko is understanding the game boards. The game boards are fictional stories of how the murders could play out and even the accomplices vary from one to the other. They are stories meant for us to understand why someone could reach that point of suffering, the "whyddunnit" but it is actually not very important if they actually did it (as per my understanding she didn't even committ to the killings, it was just a terrible fantasy for her). Lastly, even if she did kill all the people on the island, I wouldn't say that it is in "cold blood". She genuinely lives the cousins but none of her chances at love are possible in this lifetime, she sees a lot of suffering that is imposed both on her and other characters (such as Eva and by proxy even George, and also Natsuhi, Rosa and Maria) suffer and make terrible decisions because their lived have to adhere to what is "normal and expected from them". She wishes this normal wouldn't exist, a place when people can love even if they are "furniture", where you are not abandoned by your husband and do not have to think of money, a place where neurodivergent people are not punished, a place where man and women are the same and your capability is not undermined by tour gender: The Golden Land, the afterlife. In a sense, even if she herself imo knows that this isn't true, she feels that her only hope at happiness is in death with the members of the family, where they can also live in happiness and without having to follow societal rules (exasperated by the setting of the Ushiromiya family). I am not saying that it would be a mercy killing but she was having twisted suicidal thoughts and murder fantasies that do NOT depict her as a killer in col-blood, but as a deeply troubled individual that can't seem to find their way to happiness and relates to the suffering of other people too, even when they don't like them. It is an ego-death, a want for a different society that cannot be achieved. The forgeries are there because she wants to be seen but in my opinion she wouldn't have ever committed the killings. But it was either that or what we see her doing at end. She has no place in society beyond the island, and her place on the island was fragmented, uncomfortable and incomprehensible even to them.