The story angle: his "dark" path leads towards lawful evil.
The D&D morality stickler: as a vampire spawn, he's as good as evil to start with.
The psychoanalysis attempt: EA Astarion sold, among others, literal children into slavery.
He only became a vampire spawn because
the minority population he exploited and oppressed a little *too* much took their righteous revenge. If he could justify this as a naive, yet corrupt magistrate, he can justify it now. Admitting slavery is wrong means admitting his actions were wrong, means admitting his death was, possibly even, deserved. Astarion is very salty about dying.
