I'm not entirely sure I'd say I'm pulled to the dark side with Astarion,
I play my Tav's quite Chaotically to begin with which is why I always get either Gale or Astarion being my in game Partner usually dumping Gale .

I'm generally in real life quite chaotic my bedroom and wardrobes would show you my chaos so being drawn to Astarion as my first game(now 20 games later) partner was just a natural gameplay move
I also have episodes of sarcasm and gallows humor
you know the memes of a girl who has two sides of her personality cute things vs gothic darkness that is me.

The main point of his story is certainly on the dark and disturbed path SA and Slavery the ideal theory in my opinion is redeemed dark urge/ redeemed Astarion is the correct path for him they both learn to overcome their pasts ideally I want to find a sun solution for him but both redeemers probably suit the night .
now don't get me wrong I liked Ascended Astarion until the Tav's face issues in patch 6 my evil durge really shouldn't be scared of some upstart vampire lord. (she'd probably bite his head off in slayer form or using their tail as a stake as they do when casting the butler into the fire place)

Improving the path is being able to reassure Astarion in either case that he means the world to you and reaffirm that with him "I don't need to be your spawn to rule Toril with you " "I am not afraid of casting myself into the dark with you we have each other after all "

in the scene where durge has to resist killing Astarion because you failed to kill Isobel the butler says
"He is so afraid, so, so afraid of everyone besides you who he ought to fear the most"
"You like him for more than his looks, but he will never believe that"

The morning after in the thankyou for not killing me conversation he states he is worried about you more than him you've given him something to care about .
I think Act 2 is his most significant area to learn about him and how he actually feels about you and himself the Act 3 part after Cazador there really is no further character development you either Ascend him or not so that patch 6 addition is just so OOC it's awful.

We can always dispute if he's lost his soul in the rite however isn't the soul what drives the love connection how one is as a person etc..
that losing his soul is completely possible if you Ascend him he really ought to dump you the next moment
however he does not and still seems to say nice things to Tav "my consort , eternal lovers forever"
yes he is possessive and selfish however he shows that earlier in the story too before Cazador
small snippets of his conversation and behaviour show this

if you talk to him last in camp , he aims to take the leader of the group as soon as possible using his methods of seduction , you turn him down he gets quite antsy post ascension he can fall into the yandere category.
choosing someone other than him he seems to get quite jealous though he will accept it in the end.

"wyll is your typical prince type if you get bored he will always be up for a sordid affair"

So I really don't think I'm drawn into the dark with him though I'm very drawn to him.