What exactly is it that makes Astarion evil? I mean, specifically?
What does he do, outside of player control, to make him objectively evil?
He does pull a knife on you when your back is turned and try to exsanguinate you while you sleep.
Pulling a knife doesn't make him evil, right? He's worried and in danger, just crashed from a nautiloid, has no idea who we are... Do you really think that qualifies as evil?
The blood drinking... it's certainly doesn't strike me as good, but I'm not under the impression that he had any intention of draining Tav dry. Just that he had been forced into his state as a vampire and enslaved for two hundred years without being able to satisfy his intense biological need to drink the blood of something other than a rat. In fact, he seems somewhat conflicted about it.
I mean, I certainly wouldn’t pull a knife on a random passerby. If I thought they were a threat, I’d hide. I definitely wouldn’t wave them over and get them to turn their back on me.
As for blood drinking, vampires are often associated with sex because the penetration of fangs is a symbolic sexual act. So doing that without consent… hmmm. Not a good look for old Astarion.