I'm seeing lots of speculation in the wake of the unwanted guest about whether absorbing Loveday's soul is what made Cytherea snap and sure, I see the theory, but personally as a cancer survivor I have never ever questioned why Cytherea snapped

Cancer is an existentially terrifying, often debilitatingly painful and/or exhausting illness, that, if not cured, takes over your body bit by bit, gets into the highly essential bits which increases the suffering and/or fundamentally changes you as a person, makes you extremely vulnerable and dependent on others, and almost inevitably kills you unless you get it when you're old enough and die of old age first instead. Its treatment is often just as painful and exhausting as the illness, or even more so, and doesn't always work. When you have cancer, there are two ways out: being cured (preferable) or, when that is not an option, deciding for yourself when you have reached the point where the suffering is so bad and the outlook so non existent that you would rather die now rather than later after even more and worse suffering.

And John took both of those options away from Cytherea and from her entire line of descendants. When he had the option to cure them all all along. Idk about y'all but the revelation in Nona that John could cure cancer was the number one earth shattering realisation for me. He did this to her, and to the entire Seventh House, on purpose.

This is the first giant betrayal to me. For generation after generation, for TEN THOUSAND YEARS, this man let the heirs of the Seventh House be sick for functionally their entire lives, likely starting in childhood, go through an incalculable amount of painful and exhausting experimental treatments bc he didn't even bring modern medicine into his New Order, and die in their twenties or thirties at best, when he could have STOPPED THIS ALL ALONG with little more effort than snapping his fingers.

Second big betrayal is towards Cytherea herself, but basically the same point : he could have cured her at any time. Before she became Lyctor, possibly, since we're not sure how static Lyctor bodies are, but Mercy's powers, Harrow's lobotomy and Ianthe's arm suggest that it would have been an option afterwards too. And he didn't. He let her have cancer for TEN THOUSAND years without curing her. And he calls himself her friend. Absolutely fuck that bastard.

Third big betrayal is the same betrayal that he inflicts on all the other Lyctors, but imo worse bc of Cytherea's illness. It seems from the books that Lyctors are, if obv not functionally immortal, at least Very Difficult to kill. Consequently, John demands Cytherea's loyalty not only in the form of killing the person she loves the most in the world, but in the very same act, in the form of cutting off the One escape route she has left out of the suffering he's purposefully keeping her in. In short, he takes the one person away from her who she perhaps doesn't resent depending on and, in the same act, makes it A Lot more difficult for her to choose euthanasia. (There's meta in this about the deeply realistic and also Terrifying ableism of John "admits openly that he'll pay any price so the people he loves can't leave him" making his sick friend dependent on him by keeping her sick, taking away her (arguable) main caretaker and cutting her off from the option of leaving him by dying.)

And at the same time that the other Lyctors realise John's betrayal re: their cavaliers, Cytherea potentially realises ALL OF THIS. TEN THOUSAND years of suffering, of seeing her House suffer, on top of losing the person she cares about the most, and ALL OF IT was avoidable, and not just avoidable but EASY TO AVOID? Is it any wonder she went on a rampage to bring down John and everything he cared about and had worked towards, and that she didn't care about dying at the end (or, perhaps, was even counting on that outcome)? I had cancer (as an aware adult) for a year in the best imaginable circumstances and am still fucked up about almost a decade later. After TEN THOUSAND YEARS and learning there had been another option all along, I'd have done WAY WORSE than Cytherea. I really don't think she needed to absorb anyone's soul to get there. It is, in my opinion, a deeply understandable and realistic reaction.