There is no proof yet that simply possessing the necromancy tome makes a person more powerful than a collective group of people, and if it can, one would need to master it first which few could. Any and all cooperation Astarion does is for his own survival. Even if he genuinely cared not to kill his comrades, that one thing does not preclude him from being evil. That kind of logic is silly. It's like saying, "Yeah, this guy is a depraved serial killer, but he loves to pet puppies in his spare time--he's chaotic at worst!" A smidge of good does not cancel out evil, and it does even less so if the good is for selfish reasons.
This is why I've been saying that if Larian does give Astarion a "redemption" arc (shifting to neutral at best; that's as far as I can suspend my disbelief), it likely would feel shallow and unearned because Astarion was evil and corrupt in his past life too, and to change for the better requires a genuine want to do so and the ability to feel shame, which I've seen no evidence of in his case. He knows the right words to say (although he says them so disingenuously that I'm amazed our PCs don't have the option to call him out on it) to further his survival. Unless Larian's rewrites didn't stop with Wyll, and they give Astarion a significantly less evil backstory, it's just going to feel out of place and obviously done for the fans with I-can-fix-him fantasies. Leaves a bad taste in my mouth personally, but to each his own, I guess.
