he is quite frankly downright creepy in his delight of the misery and torture of others.
Agreed ...
BUT!
On the other hand, one cant really blame him ...
He was tordured and kept in misery for last few centuries ... its quite possible that he no longer even remember anything else. :-/
I mean priness Sisi was known for leting her servants be beated for each single hair they brush out of her head ... not bcs she was cruel, it was the way at that time ... it was perfectly normal for her. :-/
I think the only 'out' he has is that as an undead, Astarion is evil on a metaphysical level and without magical intervention he basically can't be anything else (I believe that's how it works in D&D, not certain). If we're going to say his behaviour is just a matter of environment though, then one can absolutely blame him. Being brought up in an environment can absolutely twist one's perspective and I believe there should be some grave given for such situations, but if he's a sapient being with full capacity for choice, then he's still responsible for those choices. You provide the example of Princess Sisi, but I'll point out that even during the height of the slave trade, in fact from the very beginning of it, there were abolitionists who argued against it. But that's getting into philosophy, admittedly.
One thing i agree, is that it should probably be game over if the he kills your main char. Also, any normal real life character would chop his head off 5 times already. Even before we get to that. The character is well writtern in my opinion. The only problem stems from actually being able to revive your char after they die.
Actions should have consequences. If you allow a starved and tortured vampire for centuries to suck on your blood, while he was not allowed to have human blood all this time, then it's not gonna end well. Vampires are notoriously bad at controlling their urges.
Yeah, that's about where I'm settled at this point. He's definitely a consistent character, but being able to be revived just screws up the whole thing.