But that's I also know that violates the Oath of Devotion so Larian clearly considers that a lack of compassion and, because the oath of devotion is coded as good, as an evil act. Astarian also looks gutted when you do it so you are not putting his feelings first and foremost.
BUT this is the story we have. In Larian's mind killing the 7k makes a "good" paladin fall.
I think that's because the game places emphasis on that it's actions that count and no what you are, you are a monster if you behave like one. Omeluum and Blurg might be the most convenient examples for this, as both come from monster races but are both actively working on making the Underdark - and in Omeluum's case itself - less monstrous. The more nefarious version of this might be the githyanki egg quest. The researchers want to raise egg-boy against his nature to be peaceful but they do so in a cruel way, which marks themselves as evil and he kills them for it.
The undead seem to also add a spin of clinging of the past - or the past clinging to you - on this general characterisation of the monstrous. Balthazar has mixed feelings towards his family and so he keeps them around just like his childhood bully, who is now his slave. Mayrina's husband is a most obvious representation of not letting go and clinging to the past. The vampire spawn too are more than a moral dilemma, they are Astarion's shame. He'd rather not think about them, but here they are. He says himself that it would be easier if they had been ghosts, but they are not they are real people whom he has to deal with - who are at one point under his control, under his authority.
Killing the spawn post ritual denies them the chance to be anything but monsters, in the context of consent you decide what's best for them and exert dominion over them. Anne Rice said vampire children cannot be happy, so these vampire children can never be happy, they are not allowed to decide on their own. Since the game puts a lot of emphasis on free will (or consent) this cannot be a good thing. You think Astarion can be more than a monster and has a right to "live", so by extension all these spawn do to.
For Astarion they also are his way to redemption, learning to care for them is dealing with his past. On a tangent: I don't quite understand why all the UD conversations and the UD ending for companion S!Astarion are buried under so many loopholes. It is a very cool ending for him on the friendship route and makes thematically more sense than the hero-adventurer one - he even gets to burn down Cazador's palace. It's kind of weird that in his romance, it is described as way less appealing. Maybe because it is less fun than adventuring? I don't know.
In this light, A!Astarion isn't a monster because of anything in the pact, but because he does something monstrous. He condemns 7K people to hellish slavery and confirms what he says about Cazador in act 1, that what Cazador was not wrong on principle but because he did it to Astarion. Act 3 S!Astarion would disagree.
Another tangent: The circus is an interesting place. While it's all fun on the surface and it seems to be a haven for odd people - Stoney and Boney probably would have a hard time living anywhere else and the raptor is happy - it is also most definitely a shady place. Lucretious does not even try to hide that they are exploiting their workers (it gets worse if you look into their diary) and the displacer beast also has a sorry tale to tell.
For Astarion's past: From the current state of the game, I get the impression that magistrate Astarion was a overly harsh judge, based on his general view on mercy and especially his conversation with Wyll, if you take both of them to Ansur. But that is also only a guess.
For Chamberlain Dufay: He is dead dead. He messed up and drank the wrong potion too, the real "Mysterious Elixir" - a potion of Feign Death - can still be found in his office.
For the Love Test: It depends on the character. Gale approves of telling him what his worst flaw is during the test, he thinks it's cathartic, to hear it said out loud. - He gets mad though, when you call him out for being a people pleaser and vain because he doesn't consider either of these such bad flaws.
