I think Astarion is naturally a protector. Hence the fact that he even chose to be judge so he can help people find justice. Might not be the only reason he studied law, but certainly would've been a part of it.
I think when one is in the situation Astarion was in, one would find oneself finding anything to justify one's actions, especially when one can't control them. So, Astarion must've told himself, "most people are flawed or bad in some way so it's okay if I take them to Cazador." It's their punishment for not being better people. It's fair in a twisted manner of logic.
But, that logic must've not worked when he met this innocent guy, who was a "sweet, sweet man," and a "darling". I'm sure he would've felt very protective of the boy and maybe even liked him. It would've been wrong to take him to Cazador because there is no reason. That's why he ran away.
So, I don't think he was in love with the boy. Astarion is a realist and he would've been under no illusions that he can sustain any relationship with anyone outside of the palace (or even inside it. Slavers keep slaves separate bc there's power in unity.), nor would he have had the emotional availability for love. He doesn't even the emotional availablilty for it in the main campaign until act 3 lmao. He could've had a soft spot for the man, but not love.
I think Astarion stopped caring not after the boy but after Cazador's punishment. I cannot imagine even a few hours of being inside that tomb, let alone an entire year. That has to completely change a person. It is a lot of time to panic, think, breakdown, reflect, create resolutions and completely 180° your entire perspective on life. That's when I think Astarion became as selfish as he did.
He prayed to be saved, no one did. He tried doing good by saving the boy, and he got punished severely for it. He wanted to be a magister and be on the side of justice, but he got kidnapped for it. I feel like the boy was taken anyway by some other spawn, just to spite Astarion as well, so him running away was useless too.
I think being in that tomb changed Astarion completely and that's when he decided that being helpful and 'good' is not the right thing to do, so he became who he we meet in the main campaign. Selfish, ruthless and a realist.