I think Astarion likes kicking people above him and shitting on people below him. Selfishness is key to his character, likely because because he hasn't had control over his actions for centuries. Though there is some implication that he wasn't a great person before being turned, his captivity is also probably why he has a distinct lack of empathy towards others, even people in whose position he'd hate to be (such as slaves).
He's a classic example of Chaotic Evil, he doesn't instigate much, but his approval largely seems to be based on which decisions will cause the most conflict, and what will leave him with the least responsibilities. Bonus points if you can do harm without suffering any consequences.
