It throws me that people appear to either 'ship them' (then again, you can ship anything I suppose; I shipped semolina porridge and ketchup at one point - judge me!) or fundamentally deny the possibility of intriguing if slightly unsettling character development between the two of them. Tbh, I see it as a profoundly good thing that this elf creeps me out and raises a number of conflicting sentiments regarding his personality and rationalisations; be anything but please, oh please, don't be boring!
To render him as a 'bad guy' seems like a bit of an injustice to his backstory and position in the lore (if the thoroughly under-developed backstory of the Aen Elle should ever receive elaboration). Then again, provided the lack of content in the game, I can understand why people are calling WTF (I did too at the beginning), or why paintbrushing the character with either black or white presents itself as an appealing option.
Judging by how frantically Ciri tries to reverse his curse, how she talks about him and how she ends up even regretting not having stayed with him in the cyberpunk world, I do wonder, what on earth did he have to do in order for Ciri to come to regard him as rather obviously more than a friend of shared circumstances (and apparently as more trustworthy than Yennefer - which is an entirely separate can of worms...). Quite the contrary to treating her badly (as in "not antisocially"), he treats her surprisingly well in the books (minus the overarching justification of his actions - which is complete nihilist garbage as far as empathising with human lifespan and its effect on humanity's sense of purpose is concerned). Otherwise he and Eredin could have simply taken her and extracted eggs from her or whatnot, given that assisted reproduction seems fairly well developed in their society, or at least, it should be, given their interest in genetic engineering.
He appears alone among all Aen Elle introduced to us, Auberon included, who is somewhat stunned in admiration of her because of the resemblance she bears to her ancestor and because of his unresolved feelings regarding her fate (it resembles an illness really). Eredin remarks upon it and also mocks him for it. I don't doubt that he finds the idea of 'a tainted copy' nauseating, which is why 'tru vuv vol 2' seems highly inappropriate, but his demeanour towards Ciri never strikes me as expressing outright revulsion (whether to her face or when talking about her). Quite the contrary, which is doubtless one of the sources of his frustrations. The one moment when he loses it is when Ciri rips open an old wound by very blatantly reminding him that there is absolutely nothing he can do to regain Lara v1.0 and yet as he still seems not to have moved on, he might as well make-do with the mockery that has now wandered onto his path. The amount of self-hatred that shines through in that situation is staggering, after which he calms down and hugs her. One could speculate that the game writers interpreted it as him realising on some level that it is not Ciri's fault for being what she is.
That they communicate almost entirely through Geralt throws me somewhat. Then there is that bizarre lab scene, which, as I understand got implemented rather late and was also almost cut. Ciri finds out absolutely nothing new regarding either Avallac'hs attitude, his (and the elves') stated interest in her abilities and genetics, or even, hell, that the elf is habiting some sort of hellish limbo between love and hatred because instead of his supposed destiny (Lara) he now gets allocated a copy-cat (Ciri). That there is something very skewed in his personal worldview is obvious, but it doesn't hurt to try to get inside that brain (well it does, but you know). That the elves tried to make up for that incalculable loss by trying to reproduce something akin to Elder Blood is, though de facto new info, still the same theme. And then there's that elf lady whom Avallac'h dismisses out of hand but who upsets Ciri - sounds more of a personal issue to her while being a plot device of 'aroused suspicion' for the player.
That sort of 180 degree turn and really just 'new' interpretation and development of the scraps from the books deserves a hell of a lot more exposition than what was provided in the game. Otherwise their relationship is truly ooc and wtf-inducing.
As far as his character goes though; you can kind of understand where he is coming from with that mix of long life, high intellectualism, unrequited love/betrayal, predestination of his own (people batter the character about the head for dealing in eugenics, and tend to forget that he was intended to be used as a breeding horse himself), and bitterness over the mockery that fate makes of his life. Not that it makes him behave as any less of a shit but one can understand his actions without condoling them.
Take that blind obsession with the notion of predestination which has royally screwed him over on a personal level and has done the same with the alder elves in general. Under Mount Gorgon, he hesitates when noting to Geralt: "...someone else will help her now. Surely you are not so arrogant to think that this girl is associated only with your destiny?" After/during which him and Eredin willy-nilly proceed to manipulate Ciri's dreams and the world so that Ciri would find her way to Tor Zireael. When you question Avallac'h on the ship he denies having imprisoned Ciri, calling her finding him her destiny. I call bullshit, but not because he couldn't sincerely believe that, but because the free will/determinist debate when brought down to the level of relationships yields such bullshit assumptions.
It is also by far the best available example of the hypocrisy of the elves who claim to be fatalists yet know full well that nothing at all will come about without correct interference at an appropriate moment. Given that the games did away with the Ciri's child subplot, I'd bet on Avallac'h being written as choosing to help Ciri as a sort of 'a-haa' moment in which he realises that if he sits around 'doing nothing' as Auberon was wont to do, he would surrender his role in predestination to Eredin - his polar opposite with whom he never fully got along, whatever their common aims. At least that's how I ended up reading it after taking for granted that the free will/determinism debate (i.e. destiny) was central to everything in the lore from interpersonal relationships to fantasy physics.
I don't deny that he is a controlling, twisted, secretive and bitter old elf; he is all of those things, which is what makes him so entertaining, and would hence turn him into a fantastic antagonist (mind, not a simplistic "enemy") if there ever was any Ciri related content ever again. Dreading simplifications, I merely regret that the aen elle subplot never received any further development (given that it would have been a seriously interesting addition to the meager bits we gathered from the books).