No, Witcher ending is the "good" ending and it's also the most Sapkowski like ending, so I would consider it the most canon one.
Dude, just stop it, you obviously have no fucking clue about how mental illnesses work. She was clearly traumatized and probably had one or several personality disorders due to her fate. And yes, she was raped by the Knight, she says so literally, but nice try wiggling out of that in your earlier post.
It's obvious, that mentally she still is the abused and wronged little girl and all she wants is to get revenge for that. But if you actually take the time to talk to her, you can feel that she's opening up, the little girl is starting to develop and get out of her protective "shell" (as in her mind would have built up walls around her feelings to prevent her from having to deal with the trauma she experienced and thus prevented her from actually getting in connection with her feelings).
So, no, she's not a bad person, she's a hurt little girl that never grew up, because of the way she was treated. And the good ending is you helping her in starting to get closure by forgiving Anna and thus getting on her way to working through her issues.
I already gave you my point of view. She is not mentally ill. She has at worst some kind of trauma and at best is basically just a bitter bitch. Furthermore her trauma started 20 years ago. 20 years in which she could have gotten help, simply worked through it or gotten some perspective on it all. She didn't, she turned to crime and became a violent criminal way before she came back to Toussaint. Do we explain that with trauma as well? That she chose the life of a criminal ring leader, terrorizing people, probably killing a few or having them killed, robbing people etc. way before she ever got back to Toussaint? I wonder how someone who has been allegedly traumatized, as you claim, by being raped and forced from her home justify in any way to basically do the same to parties who are in a similar if not worse situation, since before she came back to Toussaint I strongly doubt she only chose to rob the rich.
And no she didn't say raped. She said word for word he abused her. She did not say raped, violated, sexually abused or whatever. She said abused. Check the definition of abuse. It might as well be physical punishment or simple physical violence which, in this situation, seems far more likely to me because even in the description of the misdeeds in Syanna's letters to Dettlaff there is no mention of rape or anything coming close:
Crespi was considered dishonourable and a rogue. No indication of any wrong doing there she mentiones he beat her with a horsewhip after she tried escaping.
Du Lac was described as an idiot and being unjust, again that ties in fairly well with what she claims he did to her which is deny her food and abuse her. But she says abuse, not rape, not sexual abuse, not violate her, not fuck her, plow her or whatever. Abuse. Given that she indicated him for murder because his unjust judgements caused people to lose their lifes I personally still think it way more likely we are talking about corporal punishment and physical abuse.
De La Croix and Peyrac-Peyran basiscally were also accused of beind dishonorable and Peyrac-Peyran of being a coward. Probably because Syanna blamed them for not uttering a single word in her defense and for leaving her in the woods in a thin dress during winter.
So there is no evidence. You can watch the conversation she has with Geralt here:
https://www.youtube.com/watch?v=gKX_WqROCtU from 9:45 onwards.
Furthermore mental illness? Do you know how many women are raped, sexuall molested and beaten? Do you know how few of those women, if any, become complete sociopaths and turn to crime and running a criminal syndicate? Do you know how few even defend themselves or kill their tormentor if they have the chance? It usually takes them years of constant abuse to snap. Yet you claim a girl which, up to that point, had a fairly sheltered and well provided life and was happy enough tormenting other people before all that happens is so traumatized by all that that she loses herself for 20 years and even plans killing her sister who is in no way responsible for what befall her, being an child of 12 years old at the time? That is just rubbish.
More importantly we are talking about an episode. She wasn't held in a cage for 30 years and constantly beaten and molested. She probably spend a week at most with the knights, if that, when they escorted her out of the Duchy and left her in the border lands to fend for herself. So, besides being examined by some sorcerers she basically got her ass powdered between ages 0 and 15, then got thrown out, had 1-2 horrible weeks, met some bandits who didn't molest her, took her in and became their leader, terrorized common folk with her gang for 20 odd years and then comes back to murder people, wage war on the common folk (because that is what happens when you try to overthrow a sitting government) and kill her sister? Yeah, she sure sounds like a little girl... a spoilt little brat who doesn't give a shit because all she thinks is: mine, mine, mine, mine, mine, mine, all mine, fuck everyone, all mine.
Seriously if the women in these games need to have some sort of mental illness or complex so you can find them attractive and get vested in them then by all means... enjoy her but please don't spout this nonsense. Do you know what the difference between murder and man slaughter is? It's intent. The difference between murder 1 and murder 2? It's the planning and calculation. In Germany that is called base motives and special insidiousness which negates any form of mental illness, trauma or what so ever. This is a plan she has been hatching for years. The motives are clear, she even employs people just for this, she prepares it and even worse uses the defenselessness of the victims. Peyrac-Peyran was not dressed in armor and did not have a weapon on him. Another man was sleeping, a third was probably drunk or half drunk and the fourth was basically letting his guard down to a close personal friend. Anna Henrietta did not expect anything at all. If you do not talk Syanna out of it she will even go through with killing her sister when her sister asks for forgiveness and shows remorse, she uses an embrace to kill her. That is icecold. We are not talking about a little girl here or a mental illness. We are talking about a cold, calculated revenge killing with a base motive, enriching herself and getting controll of the duchy. For no matter how she tries to bend it, the end game would be to take her rightful place, as she was planning a rebellion which she told the owner of the castle she had comandeered and expressed several times when talking about her lost birth right.
So what do you want more? 20 years of planning, dozens of people involved, manipulation of a close friend, absolute cold precision when going for the kills and the endgame being to take Anna Henrietta's place. That is murder 1 no matter where you go in this world. That get's you life in prison with no parole or the death sentence. You can not claim mental illness in any court if it is shown without reasonable doubt that the accused had her faculties in order so much that she was able to plan and orchestrate a detailed plan to kill someone for personal gain. It's even worse when you can show that the person had the intelligence and mental faculties to actually form a criminal syndicate and lead it and obviously achieve her goals.
No matter how you twist it, in no country in the world would she go free or be declared insane, even temporarily.