I don't think that as a Vengeance paladin we'd be obligated to kill him.
Now when you say it ... me neither.
I think that scene should (and would) work well enough if it would be the same as with Crusher ... just kick his butt well enough to show him his place.
It just frustrates me beyond reasons when NPCs are systematicly made invincible, and those assholes are using that to rub your face in it. -_-
//Edit:
At this point, the worst we can say about Astarion is that he's maybe a liar and definitely a suspicious asshole.
Quite soft and not really acurate description in my opinion ... we can disagree ofcourse.
But no matter what he just escaped, he is a person willing to attack, threaten and possibly kill completely random stranger ... that IS dangerous and quite Evil behaviour. O_o
I mean, i completely understand that every Paladin have their own measurement ...
But this is mine ... and what else can i say but:
"And I will strike down upon thee with great vengeance and furious anger those who attempt to poison and destroy my brothers. And you will know I am the Lord when I lay my vengeance upon you!"