Astarion and Halsin's traumas were meant to be foils
When characters are foils, there are two components: first, there's a shared background, event, personality trait, etc. But how the characters act from then on are diametrically opposed, allowing us an insight into the various ways people can act or respond to one core "element". In this case, I would argue that Halsin and Astarion are meant to be foils in their responses to sexual slavery.
Both Astarion and Halsin were denied their freedom and agency, raped and abused. Both were very young when this happened; Astarion was in his 30s, which is before elves reach their majority, while Halsin's age wasn't specified, but he goes out of his way to mention "youth" many times. In other words- both were young enough for this to be a formative memory for them. Both carry deep traumas from their experiences. Both are incredibly physically attractive, and allude to or outright say that their looks played a part in their captivity; Astarion was used to seduce others for Cazador, while Halsin notes that his Drow captors "took an interest in him" and saw him "as a novelty"- most likely for his looks as much as for his race. Both were enslaved by people of high social status- Cazador a wealthy influential figure in Baldur's Gate, and Halsin's captors high-ranking Drow nobles. That is what they have in common.
But their responses to their traumas are complete opposites.
First, just the nature of how they express their traumas. Astarion is loud about it. He expresses it all openly; he is traumatized. And he knows he didn't deserve what happened to him.
Halsin buries it. He pretends it was no big deal. He victim-blames himself, saying it was his fault for being a "foolhardy young Druid" intent on seeing the Underdark.
Astarion despises Cazador; he wants revenge. He will do anything to get revenge on his abuser. This need for closure is the core of Astarion's entire arc, to the point that of all the scenarios I can think of where Astarion leaves the party, most of them involve his journey to kill Cazador.
Halsin has trauma bonds to his captors. He speaks kindly of them even when describing their abuse. He says he feared for his life, but he "did some things that were less than necessary," making it sound like he was complicit in his own rape. He can't even bring himself to call them captors (except for one option in the post-patch 5 dialogue), nor himself a sex slave; instead, he was something "between a guest, prisoner, and consort."
Astarion is (in most cases) ultimately allowed closure; he kills Cazador. In the bad path, he then joins the cycle of abuse by killing the other vampires and subjugating a romanced avatar; in good scenarios, he only kills Cazador, and then has a cathartic, tearful breakdown after.
Halsin never had (or seemed to want) that kind of closure; he escaped while his captors were fighting another noble house, and his freedom was all he wanted. Whether his captors lived or not, he doesn't care. All he wanted was his own freedom.
Astarion is younger, and his trauma a shorter time ago, yet he has processed what happened more; he is both further ahead and further behind on his healing journey than Halsin.
Halsin is older, and his trauma longer ago, but he hasn't processed what happened to him; bouncing from trauma to trauma and being forced into a leadership role caused him to have to bury it. He is both further behind and further ahead on his healing journey than Astarion.
Astarion makes a point of avoiding intimacy, and he only has a few exceptions with the player. (Ascended Astarion becomes much more confident, but that's a bit different.)
Halsin is incredibly sexually open. He enjoys sex of all kinds, finding it comforting, and the only way he can openly express his emotions after having to stay in control as Archdruid all the time.
Astarion dissociates during the Drow brothel orgy. He is miserable and uncomfortable, but doesn't regret it; he needed to take the step to explore his sexuality on his terms. Even if it triggered him, he still wanted the experience, and indeed, finding what one's triggers are is an important step for many survivors.
Halsin enjoys himself during the orgy, and even seems pleased after, but then he lets the cracks show, talking about how he was held as a slave. He enjoyed the orgy during, but after, the thoughts started creeping in, as he was reminded of his captivity.
Astarion will respond to cruel player comments about Cazador with a massive hit in approval, and possibly breaking up with a romanced player, like when they say they have a kidnapping fantasy about him if he's kidnapped by the spawn in act 3.
Halsin, in the post-patch 5 dialogue options, doesn't seem to react that much even to cruel comments; when the player threatens to sell him back into slavery, all he has to say is, "you would be unwise to attempt it, trust me. In any case, the house of my captors is long-extinct." (Followed by him having an epiphany that they WERE his captors) He never gets angry at the player despite the absolute evil of this option; as with nearly every other mean thing the player says to him, he simply shrugs it off, clearly sad but brushing it off as always. Being the "bigger person", literally and metaphorically.
Astarion was left with scars all over his back, symbolizing how this is something he'll never break free from entirely.
Halsin was left with no scars, his only prominent one being from an unrelated incident, symbolizing how much work he puts in to hide his traumas.
It's understated, so a lot of players aren't going to think about it much because of this, but I think it's worth bringing up as a note on characterization!
