Astarion logic is "I was abused so I should be able to kill the people who were abused along side me in a demonic ritual sacrifice[ and the abuser]. Never heard that before. Lot of victims want to kill their abuser but the people who were abused along side them ? I don't recall hearing/reading that behavior. Their is a difference between having a nasty attitude from being abused and maybe violent against potential aggressors and planning to be a family annihilator [which isnt caused by abuse.]
I mean, yes, you haven't heard about ritual sacrifices that involve 7k people irl because they don't happen. In stories they tend to do over-the-top metaphors for things. Shadowheart's story is about having your identity eroded by an abuser/cult, not about an actual deity of Loss giving you amnesia. Same with Astarion. It's not literally about traumatized people wanting to kill those around them, it's just the way the game is conveying the idea it's trying to more effectively, whether you think it works or not is up to you of course.
On that point, I think you're wrong about abused people not being able to want to harm others other than their abuser. This is extremely common and both heard of in real life and fiction. Astarion's story is most closely trying to reflect that of familial cycles, so imagine it as the well known tried and true "dad abuses son, son grows up into a dad, new dad abuses his new son, new son becomes a dad once again, etc etc".
And even without that I think it's extremely common for some victims to try to gain power over others by hurting them because they feel powerless, and they've been hurt. Astarion's killing of the 7k also seems to stand for him trying to evade and destroy any evidence of his own abuse, so I think that also makes sense as a parallel to how real victims might try to avoid facing what's happened to them and "run" instead.
