From my playthough I didn't get the impression that he would like slavery at all. Time and again he told my Tav, (and my checks from his thoughts and/or insight narrated it, too) that his main motivation is
. I believe that's genuine, and it would make sense with his storyline.
He doesn't believe that the world is fair or that the strong ones would save anyone. He gets angry when Tav proposes that there is selfless heroism in the world. That's a very understandable point of view for a survivor and that reaction is something that you can actually hear from many real-life survivors. The world has to be hard and unfair (and people denying that have to be just naive), because otherwise there is no point in why the world let you suffer like that. Astarion has a line where he says exactly that
(something about how the strong do not care about the weak, because if they did, why did no one ever come for him... and in separate occasion that gods don't care about us, because he prayed to them all and none helped him).
So from that I think it's understandable that he doesn't actively stand against slavery, but instead takes it for granted - as something that just is, always has been and will be. I noticed that this reaction did diminish during the game and he stopped disliking us saving everyone all the time. Maybe there is hope for him, in the end.
Instead the reason for him liking leaving random people in dire situations and disliking engaging in a fight for them seems to be that he doesn't want to risk himself (and later on you, if you are his partner).
