In a way, Astarion moves on from trusting no one to reluctantly trusting the player in Act 1 (given reason: shared interest in survival). His world view isn't changing, but his patterns of behaviour are.
The ascension route affirms to him that the only way to trust is to have the power to enforce your desired results. Most people see a commentary on the cycle of abuse, but I see one on the hunger for power. This includes the reasons to want power -- avoiding victimisation, self fulfillment, even healing.
How people see the ascension route is influenced massively by how the view power. There are people who straight up refuse to believe it's the "bad" ending. There are also people who see Astarion as either a fullfilled or unfulfilled, *unrealized* egomaniac. To them, the difference between the routes is negligible. There are also people who see spawn Astarion as a good guy. It's really rather curious.
Personally, as a concept in lore, vampire spawn are automatically and inherently evil. Whatever their alignment before the change, it doesn't matter. I think Larian brought that to screen very well, though there are people who disagree with me on that Astarion or anyone is meant to have an alignment. He is like the Durge in that there is complexity between nature and nurture. His nature is, I believe, neutral evil though. You can encourage him to be good, but it will never fully go away.
