Astarion is a vampire, and therefore literally evil. I just don't see how that's supposed to be a counterargument to adding depth to his backstory.

When people "rationalise", it's: "spending some odd hundred+ years as a slave gives you trust issues". Not "therefore he did nothing wrong ever".

You can not condone the actions of a fictional character without hating him. It's the same deal as for why people like villains.