In the Grymforge, Astarion (former slave) disapproves of Tav helping the gnome slaves save their other trapped friends, and approves of letting Nere choke them to death.
This part actualy makes sence, since Astarion is ... well, asshole.

We can obviously speculate about how much of that attitude comes from himself, and how much of it comes from the fact that he was tortured and humiliated by Cazador in few last centuries (aka longer that anyone of us would even remember) ... and however odd that may seem, he really dont know anything else anymore ... and therefore his attitude towards any slaves dont come from sadistic pleasure in tormet of others, but from the fact that it seems perfectly normal to him that weak suffers.
(This is actualy well documented thing.)
Astarion can stay true to his character concept but let Tav react to it - just as Tav can now react to the vampire hunter scene.
+1,000,000 ...
Not just in this case ... Tav really need to grow some balls (regardless of his or hers gender

) and get a LOT more options to give companions taste of their own medicine. :-/
Its not just Astarion who would deserve it.