EDIT: (link) I think Alys sent Aemond to his death to protect herself or at least didn't stop him because there was no world where she wasn't in constant danger beside him, even if he took her to KL. The people around the greens or a part of them (especially Alicent, this conservative woman who, like her show counterpart, thinks about appearances), would try to remove or use her.
I don't think she was as attached to him as he was to her, even if there was some sort of reluctant affection from her (not quite full-fledged Stockholm syndrome, since I think she always maintained her eye on the prize: self-preservation).
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A)
Alys could send Aemond to his death because he made her his war prize and immediately began having sex with her after destroying her means of living (look at point “B”, there’s more).
Alys is referred to many times as Aemond’s “bedmate”. Her purpose is sexual.
No trueborn Strong was spared, nor any bastard save...oddly...Alys Rivers. Though the wet nurse was twice his age (thrice, if we put our trust in Mushroom), Prince Aemond had taken her into his bed as a prize of war.
(”Rhaenyra Triumphant”)
So here is this guy who has literally made a pile of heads out of the children and adults you have seen or interacted with, even causally, for years. It's still traumatic to see them die violently in front of your very eyes, or know how they died. Even if you didn't love them, you knew them. Plus, if we believe that Aemond even killed kids, that would induce a lot of fear and panic in those left behind, even if they didn't like kids. Women and girls get raped by occupying soldiers often in all sorts and periods of war, too. But otherwise, it's usually a heartbreaking thing to know/witness: the violent murders of children. No one good or sane approves or ignores to see the most vulnerable people get killed or abused.
She also seems to show some compassion for that messenger that Aemond was going to beat up. So I imagine she would have been horrified by the live-extermination before. I see little reason why Alys would not be affected similarly as that is the usual response to such, and if you argue otherwise you need very good evidence in the text/context.
Again, Aemond murders these male Strongs -- man and child-- openly. For everyone left to witness...
Do you actually think that Alys would have seen this and thought, "hmmm this guy will take good care of me (specifically in the moment and right after when he chooses her out of others to keep) and respect me or care of my feelings beyond his own"? You’d have to present a good reason and evidence to suggest how she’d know of his coming, which admittedly reveals how little we know of Alys and her true abilities or even if she was a witch or if she was just Aemond’s “type”.
Here is another, earlier example of what war prizes actually were, as opposed to a legitimate paramour or mistress:
Bold Jon Roxton became enamored of the beautiful Lady Sharis Footly, the wife of the Lord of Tumbleton, and claimed her as a “prize of war.”. When her lord husband protested, Ser Jon cut him nigh in two with Orphan Maker, saying, “She can make widows too,” as he tore the gown from the weeping Lady Sharis.
(”Rhaenyra Triumphant”)
And Ian Plate writes this in his article:
The treatment of women as objects, used to mark male prestige, appears in our earliest extant Greek literature, Homer's Iliad. Here, women are valued as prizes in competition between men, awarded to acknowledge relative male prowess.
There is no consent here, at all. None. And little guarantee of safety.
B)
Never said or claimed that she DEFINITELY sent Aemond to his death for her family. Never even said she cared or loved them.
I said she materially depended on the Strongs for her own living.
She may not have loved them per se but it’s possible that she would still feel like she lost some sort of "home" as well as her means for living and economic support -- as she lived with them all her life, had memories, and grew up with them in her formative years. To have that gone in such violence RIGHT IN FRONT OF HER FACE will still discombobulate your sense of security as I already said, so that's another motivation against Aemond.
Though breastfeeding was not always a respected job and she would not have been as respected as the bastard closer in relation, she did relatively better as their wetnurse than if she married outside and became a “serf”’s wife. Which is probably why she even stayed for so long, as we see by her age (not old, but she never chose to marry and have kids elsewhere and she was estimated to be in her late 30s to mid 40s...which is old to not have a family to these medieval persons).
Aemond, destroying her means to live, thus made her much more vulnerable, giving her a less stable life. War prizes depend on their captor and their captor alone. (I go into Alys’ vulnerability if she had gone with him to King’s Landing or if Alicent knew about her below). That is enough for a person to hate him even without the war prize event.
But add the war prize to it, with her history of losing children (wet-nurses were women who weaned their child too early or lost their infants very early...hence their ability to breastfeed), forcing her to get pregnant again and scrounging up her past miscarriages or stillbirths...Alys sees Aemond murders this Strong kids (even if she weren't close to them), and remembers her own loss....trauma reborn.
There’s a lot to hate and fear Aemond for.
C)
Official, "true" paramours and other mistresses historically are not usually war prizes. War prizes are those who were captured and taken as reward for an individual’ military prowess and conquest, as I’ve already stated.
While paramours, mistresses, and war prizes all occupy this strange space of “lover of a lord/warrior under his sociopolitical authority and dependent on him”, because war prizes literally have no choice in the face of harm or violence after being captured, they are not like other mistresses or paramours. They are not “free”. Their lives don’t really matter as much to anyone but their captors, who can decide when they live and when they die. Mistresses and paramours like Samantha Tarly, Ellaria Sand, Barbra Bracken, Bellegere Otherys, etc. have a lot more freedom since they weren’t taken as prizes during/after the captor’s victory and usually have some backing from families and even husbands. Some money independent from their lovers to fall back on, even if they received some moneys from said lover. They are protected by customs and/or self-allocated resources and connections.
Thus, they would very likely and often not be treated as well as the average paramour/mistress, who weren't bonded to the lord in a war/violence/reward context.
Of course, having a child would make Alys and other war prizes’ prospects a bit more politically “better”, since lords are still expected to take care of their bastards or look out for their well being in some capacity. However, realistically, this puts both Alys and her kid in more danger, with that kid now traced back to Aemond.
What would the others (Otto, Aegon, Alicent) have thought and done about this child to maintain their images and royal image even if it’s a man/Aemond and not Helaena having a bastard? By how they neglect Aegon’s bastards, I’d imagine they’d try to get rid of Alys’ child. Maybe not violently, but...it's just not in Alys' interests to be perceived as a nuisance or even as a "thing" that creates a flaw in the greens' image or nettle Aemond's mother. Viserys II forced Aegon IV's mistress, Megette, to leave even after birthing Aegon's first set of kids, 3 daughters. Megette was eventually beaten to death by her husband. Daeron is one of those who supposedly looks at Jon Roxton in “horror” when he rapes the Lady of Tumbleton and kills her husband. Which happens presumably before Aemond takes Alys. Which means that this image of Hightower religious and moral purity is tainted by Aemond’s action. What if the Hightowers had won, Aemond came back with Alys to court? Especially with the rumors of Alys being a witch and Alicent/the Hightowers being so religious or seeming so....
You see the issue here, for Alys? It’s actually best if Aemond dies and she gets to run off, even if she didn't know what the other greens were like.
And that's the issue in her perspective as a war prize: the future is a lot more unpredictable and likelier to be dangerous than it was before. And we still don't know how Alys's probable visions work...does she just get them out of the blue, or can she focus and see events yet to happen? Both? Can she ever really trust those visions?